scudo: Allow TBI to be disabled on Linux with a macro.

[pcc]

Summary

Android's native bridge (i.e. AArch64 emulator) doesn't support TBI so
we need a way to disable TBI on Linux when targeting the native bridge.

This can also be used to test the no-TBI code path on Linux (currently
only used on Fuchsia), or make Scudo compatible with very old
(pre-commit d50240a5f6ceaf690a77b0fccb17be51cfa151c2 from June 2013)
Linux kernels that do not enable TBI.

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

 void setRandomTag(void *Ptr, uptr Size, uptr ExcludeMask, uptr *TaggedBegin,
                   uptr *TaggedEnd);
 
 #if defined(__aarch64__) || defined(SCUDO_FUZZ)
 
 // We assume that Top-Byte Ignore is enabled if the architecture supports memory
 // tagging. Not all operating systems enable TBI, so we only claim architectural
 // support for memory tagging if the operating system enables TBI.
-#if SCUDO_LINUX
+#if SCUDO_LINUX && !defined(SCUDO_DISABLE_TBI)
 inline constexpr bool archSupportsMemoryTagging() { return true; }
 #else
 inline constexpr bool archSupportsMemoryTagging() { return false; }
 #endif
 
 inline constexpr uptr archMemoryTagGranuleSize() { return 16; }
 
 inline uptr untagPointer(uptr Ptr) { return Ptr & ((1ULL << 56) - 1); }