[compiler-rt] Allow 64-bit sanitizer allocator to be used if using RISCV64 and Fuchsia

[leonardchan]

Summary

This way, Fuchsia can use the 64-bit allocator settings in D151157 without changing the default behavior for others.

[mcgrathr]

Certainly all Fuchsia cases (unless/until we support ILP32) should use the 64-bit allocator.

compiler-rt/lib/sanitizer_common/sanitizer_platform.h

 #endif
 
 // By default we allow to use SizeClassAllocator64 on 64-bit platform.
 // But in some cases SizeClassAllocator64 does not work well and we need to
 // fallback to SizeClassAllocator32.
 // For such platforms build this code with -DSANITIZER_CAN_USE_ALLOCATOR64=0 or
 // change the definition of SANITIZER_CAN_USE_ALLOCATOR64 here.
 #ifndef SANITIZER_CAN_USE_ALLOCATOR64
-#  if SANITIZER_RISCV64 || SANITIZER_IOS
+#  if (SANITIZER_RISCV64 && !SANITIZER_FUCHSIA) || SANITIZER_IOS
 #    define SANITIZER_CAN_USE_ALLOCATOR64 0
 #  elif defined(__mips64) || defined(__hexagon__)
 #    define SANITIZER_CAN_USE_ALLOCATOR64 0
 #  else
 #    define SANITIZER_CAN_USE_ALLOCATOR64 (SANITIZER_WORDSIZE == 64)
 #  endif
 #endif