libretro: build the Switch core, and turn the iOS 9 job back off (#1180)

* ci: turn the iOS 9 job back off, with what it said on the way out

It was the one of the five I could not try before pushing, and one run was
enough. It compiles the pinned commit and then fails to link:

    Undefined symbols for architecture armv7:
      "std::bad_optional_access::~bad_optional_access()"
      "vtable for std::bad_variant_access"
      "vtable for std::bad_optional_access"

Those arrived in Apple's libc++ with iOS 12 and 32-bit iOS ended at 10, so no
armv7 slice of any SDK carries them. Supplying them from the core does work -
but it only moves the failure, because building the tip for armv7 stops before
that, inside libc++:

    __format/formatter_floating_point.h:74: error: 'to_chars' is unavailable:
    introduced in iOS 16.3

std::format with a floating-point argument reaches the floating-point half of
<charconv>, which Apple shipped in iOS 16.3 / macOS 13.3 / tvOS 16.3. Every
Apple target here is built below that, which is also what OSX_BUILD_COMMIT is
working around - the Apple jobs build a commit from before that code arrived,
now some 670 behind.

So the job goes back into a comment with both errors in it, rather than being
left red for whoever looks at the pipeline next. The other four targets from
the last change are green.

* ci: build the Switch core, which the recipe has been ready for

The libnx template has been in the include list since .gitlab-ci.yml was
written, the job under it has been commented out for just as long, and the
Makefile has had a platform=libnx branch the whole time - so the Switch has
never had a core built from this tree.

It does not build as it stands: -std=c++23 defines __STRICT_ANSI__, newlib
reads that as "no POSIX names", and libretro.cxx calls strdup:

    ../../os/libretro/libretro.cxx:140:18: error: 'strdup' was not declared in
    this scope; did you mean 'strcmp'?

-D_GNU_SOURCE in the libnx flags is enough - devkitA64 has the function, the
header simply will not declare it otherwise.

Built with devkitA64 before turning the job on, so what lands on the buildbot
is a job that passes rather than one that finds this out in a nightly.

.gitlab-ci.yml

@@ -205,16 +205,40 @@ libretro-build-ios-arm64:
     - git checkout "$OSX_BUILD_COMMIT"
 
 # iOS (armv7, iOS 9)
-libretro-build-ios9:
-  tags:
-    - mac-apple-silicon
-  extends:
-    - .libretro-ios9-make-default
-    - .core-defs
-  before_script:
-    - export NUMPROC=$((1 + $(sysctl -n hw.ncpu)/5))
-    - git fetch origin
-    - git checkout "$OSX_BUILD_COMMIT"
+#
+# Off again after one run, and off for a reason that is worth writing down.
+#
+# It compiles the pinned commit and then does not link:
+#
+#     Undefined symbols for architecture armv7:
+#       "std::bad_optional_access::~bad_optional_access()"
+#       "vtable for std::bad_variant_access"
+#       "vtable for std::bad_optional_access"
+#
+# Those members came to Apple's libc++ with iOS 12 and 32-bit iOS stopped at
+# 10, so no armv7 slice of any SDK has them. That part can be carried in the
+# core, but it is not the wall. Building the tip for armv7 stops earlier, in
+# libc++ itself:
+#
+#     __format/formatter_floating_point.h:74: error: 'to_chars' is unavailable:
+#     introduced in iOS 16.3
+#
+# std::format with a floating-point argument reaches the floating-point half of
+# <charconv>, which Apple shipped in iOS 16.3 / macOS 13.3 / tvOS 16.3 - above
+# every deployment minimum used here, and the reason OSX_BUILD_COMMIT exists at
+# all. So this stays a comment until floating-point formatting stops needing a
+# library version armv7 can never have; the include above stays with it.
+#
+# libretro-build-ios9:
+#   tags:
+#     - mac-apple-silicon
+#   extends:
+#     - .libretro-ios9-make-default
+#     - .core-defs
+#   before_script:
+#     - export NUMPROC=$((1 + $(sysctl -n hw.ncpu)/5))
+#     - git fetch origin
+#     - git checkout "$OSX_BUILD_COMMIT"
 
 # tvOS
 libretro-build-tvos-arm64:
@@ -228,10 +252,10 @@ libretro-build-tvos-arm64:
 
 ################################### CONSOLES #################################
 # Nintendo Switch
-# libretro-build-libnx-aarch64:
-#   extends:
-#     - .libretro-libnx-static-retroarch-master
-#     - .core-defs
+libretro-build-libnx-aarch64:
+  extends:
+    - .libretro-libnx-static-retroarch-master
+    - .core-defs
 
 #################################### MISC ##################################
 # webOS 32-bit

src/os/libretro/Makefile

@@ -168,7 +168,10 @@ else ifeq ($(platform), libnx)
    include $(DEVKITPRO)/libnx/switch_rules
    TARGET := $(TARGET_NAME)_libretro_$(platform).a
    DEFINES := -DSWITCH=1 -D__SWITCH__ -DARM
-   CXXFLAGS += $(DEFINES) -fPIE -I$(LIBNX)/include/ -ffunction-sections -fdata-sections -ftls-model=local-exec -specs=$(LIBNX)/switch.specs
+   # -std=c++23 defines __STRICT_ANSI__, which is what newlib reads to decide
+   # whether POSIX names exist at all - and libretro.cxx calls strdup. devkitA64
+   # has it; the header just will not declare it without this.
+   CXXFLAGS += $(DEFINES) -D_GNU_SOURCE -fPIE -I$(LIBNX)/include/ -ffunction-sections -fdata-sections -ftls-model=local-exec -specs=$(LIBNX)/switch.specs
    CXXFLAGS += -march=armv8-a -mtune=cortex-a57 -mtp=soft -mcpu=cortex-a57+crc+fp+simd -ffast-math $(ASFLAGS)
    STATIC_LINKING = 1