[sanitizer_common] Use 38-bit mmap range for Fuchsia (#69387)

46cb8d9a unconditionally changed the
mmap range to 2^48 for all riscv sanitizers. This changes the allocator
tunings for the 32-bit allocator for riscv and led to a severe
performance regression for our lsan tests. This effectively revers the
tuning change but only for Fuchsia.

Once we enable the 64-bit allocator for everything riscv, this value
will be irrelevant since it's only relevant for the 32-bit allocator.