[libc] Add a simple linux aarch64 config.

Summary:
With this change, "ninja check-libc" on linux/aarch64 succeeds.

However, all entrypoints with machine dependent implementations
have been skipped. A good number of these skipped entrypoints can
be enabled once we have aarch64 syscall support available.

Reviewers: abrachet, asteinhauser

Differential Revision: https://reviews.llvm.org/D81533

libc/config/linux/aarch64/entrypoints.txt

@@ -0,0 +1,26 @@
+set(TARGET_LIBC_ENTRYPOINTS
+    # errno.h entrypoints
+    libc.src.errno.__errno_location
+
+    # string.h entrypoints
+    libc.src.string.strlen
+)
+
+set(TARGET_LIBM_ENTRYPOINTS
+    # math.h entrypoints
+    libc.src.math.ceil
+    libc.src.math.ceilf
+    libc.src.math.cosf
+    libc.src.math.expf
+    libc.src.math.exp2f
+    libc.src.math.fabs
+    libc.src.math.fabsf
+    libc.src.math.floor
+    libc.src.math.floorf
+    libc.src.math.round
+    libc.src.math.roundf
+    libc.src.math.sincosf
+    libc.src.math.sinf
+    libc.src.math.trunc
+    libc.src.math.truncf
+)

libc/config/linux/aarch64/headers.txt

@@ -0,0 +1,5 @@
+set(PUBLIC_HEADERS
+    libc.include.errno
+    libc.include.math
+    libc.include.sys_syscall
+)