[compiler-rt] Allow Fuchsia to use 64-bit allocator for RISCV (#68343)

The allocator should be tuned well enough for Fuchsia+RISCV.

Author: PiJoules

compiler-rt/lib/sanitizer_common/sanitizer_platform.h

@@ -284,7 +284,8 @@
 // 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 || SANITIZER_DRIVERKIT
+#  if (SANITIZER_RISCV64 && !SANITIZER_FUCHSIA) || SANITIZER_IOS || \
+      SANITIZER_DRIVERKIT
 #    define SANITIZER_CAN_USE_ALLOCATOR64 0
 #  elif defined(__mips64) || defined(__hexagon__)
 #    define SANITIZER_CAN_USE_ALLOCATOR64 0