ci: add the Android x86_64 build

Held to NDK 26: x86_64 is the ABI that compiles dynarmic's x64 backend, and
its mcl headers do not survive the clang in NDK 29. arm64-v8a never reaches
that code, so it stays where it is.

.gitlab-ci.yml

@@ -81,6 +81,22 @@ android-arm64-v8a:
     ANDROID_NDK_VERSION: 29.0.14206865
     NDK_ROOT: /android-sdk-linux/ndk/$ANDROID_NDK_VERSION
 
+# Android 64-bit x86
+#
+# Held to NDK 26 on purpose. This is the ABI that compiles dynarmic's x64
+# backend, and the mcl headers it pulls in do not survive the clang in NDK 29:
+# "implicit instantiation of undefined template
+# mcl::mp::detail::lift_sequence_impl". arm64-v8a never reaches that code,
+# which is why it is fine on 29. 26.2 is also a version the template leaves
+# alone - it rewrites a request for 27.3 into 29.
+android-x86_64:
+  extends:
+    - .libretro-android-cmake-x86_64
+    - .core-defs
+  variables:
+    ANDROID_NDK_VERSION: 26.2.11394342
+    NDK_ROOT: /android-sdk-linux/ndk/$ANDROID_NDK_VERSION
+
 ################################### CONSOLES #################################
 
 # libretro-build-libnx-aarch64: