compiler-rt: correct RISC-V struct_kernel_stat64_sz

The value of struct_kernel_stat64_sz introduced by review D5021 for
RISC-V was incorrect.

Also add a __riscv_xlen == 64 conditional as the 32-bit ABI is not yet
finalized.

Submitted by: Luís Marques
Differential Revision: https://reviews.freebsd.org/D21684