Reapply "[compiler-rt] Allow Fuchsia to use 64-bit allocator for RISCV (

#68343)"

This reverts commit 37432c1.

The tunings for the lsan allocator for Fuchsia on RISCV should be
adjusted.

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