[Preprocessor] Define __GCC_HAVE_SYNC_COMPARE_AND_SWAP_16 for AArch64 (#74954)

GCC sets `#define HAVE_atomic_compare_and_swapti 1` and therefore
defines `__GCC_HAVE_SYNC_COMPARE_AND_SWAP_16`.

Clang compiles the 16-byte legacy `__sync_bool_compare_and_swap` and new
`__atomic_compare_exchange` compile to LDXP/STXP or (with LSE)
CASP{,A,L,AL}.

Link: #71883

Author: MaskRay

clang/lib/Basic/Targets/AArch64.cpp

@@ -574,11 +574,12 @@ void AArch64TargetInfo::getTargetDefines(const LangOptions &Opts,
   else if (*ArchInfo == llvm::AArch64::ARMV9_5A)
     getTargetDefinesARMV95A(Opts, Builder);
 
-  // All of the __sync_(bool|val)_compare_and_swap_(1|2|4|8) builtins work.
+  // All of the __sync_(bool|val)_compare_and_swap_(1|2|4|8|16) builtins work.
   Builder.defineMacro("__GCC_HAVE_SYNC_COMPARE_AND_SWAP_1");
   Builder.defineMacro("__GCC_HAVE_SYNC_COMPARE_AND_SWAP_2");
   Builder.defineMacro("__GCC_HAVE_SYNC_COMPARE_AND_SWAP_4");
   Builder.defineMacro("__GCC_HAVE_SYNC_COMPARE_AND_SWAP_8");
+  Builder.defineMacro("__GCC_HAVE_SYNC_COMPARE_AND_SWAP_16");
 
   // Allow detection of fast FMA support.
   Builder.defineMacro("__FP_FAST_FMA", "1");

clang/test/Preprocessor/init-aarch64.c

@@ -118,6 +118,7 @@
 // AARCH64-NEXT: #define __FP_FAST_FMAF 1
 // AARCH64-NEXT: #define __GCC_ASM_FLAG_OUTPUTS__ 1
 // AARCH64-NEXT: #define __GCC_HAVE_SYNC_COMPARE_AND_SWAP_1 1
+// AARCH64-NEXT: #define __GCC_HAVE_SYNC_COMPARE_AND_SWAP_16 1
 // AARCH64-NEXT: #define __GCC_HAVE_SYNC_COMPARE_AND_SWAP_2 1
 // AARCH64-NEXT: #define __GCC_HAVE_SYNC_COMPARE_AND_SWAP_4 1
 // AARCH64-NEXT: #define __GCC_HAVE_SYNC_COMPARE_AND_SWAP_8 1