scudo: Fix compilation for non-Linux aarch64

[mcgrathr]

Summary

Linux-specific aarch64 code was deconditionalized in commit
dfa40840e0e2fa094c5d3f441affe0785cdc8d09. This broke builds for
non-Linux aarch64 platforms.

[cryptoad]

Let's go with yours, better comments/whitespaces.

compiler-rt/lib/scudo/standalone/memtag.h

inline uint8_t extractTag(uptr Ptr) {
   (void)Ptr;
   UNREACHABLE("memory tagging not supported");
 }
 
 #endif
 
 #if defined(__aarch64__)
 
+#if SCUDO_LINUX
+
 inline bool systemSupportsMemoryTagging() {
 #ifndef HWCAP2_MTE
 #define HWCAP2_MTE (1 << 18)
 #endif
   return getauxval(AT_HWCAP2) & HWCAP2_MTE;
 }
 
 inline bool systemDetectsMemoryTagFaultsTestOnly() {

 #ifndef PR_MTE_TCF_MASK
 #define PR_MTE_TCF_MASK (3UL << PR_MTE_TCF_SHIFT)
 #endif
   return (static_cast<unsigned long>(
               prctl(PR_GET_TAGGED_ADDR_CTRL, 0, 0, 0, 0)) &
           PR_MTE_TCF_MASK) != PR_MTE_TCF_NONE;
 }
 
+#else // !SCUDO_LINUX
+
+inline bool systemSupportsMemoryTagging() { return false; }
+
+inline bool systemDetectsMemoryTagFaultsTestOnly() { return false; }
+
+#endif // SCUDO_LINUX
+
 inline void disableMemoryTagChecksTestOnly() {
   __asm__ __volatile__(".arch_extension mte; msr tco, #1");
 }
 
 inline void enableMemoryTagChecksTestOnly() {
   __asm__ __volatile__(".arch_extension mte; msr tco, #0");
 }