[compiler-rt] Go back to using the 32-bit allocator for Fuchsia+RISCV64

Due to logic in the 64-bit allocator, the smallest allocation we can
zx_vmar_allocate is 2^37 bytes, but this is too large to allocate for
the standalone lsan allocator on a 39-bit VMA. This leads to the
zx_vmar_allocate call when initially setting up the allocator to fail.
This is similar to what android experiences on a 64-bit system with a
small vma. (See sanitizer_allocator_test.cpp).

This effectively reverts f6c4808d and
has Fuchsia use the 32-bit allocator for RISCV.