Add RISC-V unaligned access

RISC-V C API Specification says that if the C compiler defines `__riscv_misaligned_fast`, then unaligned access is known to be fast.

Maybe we can update `xxhash.h` to add unaligned access for some RISC-V machines.

The related code:

https://github.com/facebook/zstd/blob/bfd8ad886f2948a8a0159a28e0aa6153611cd1c1/lib/common/xxhash.h#L2254-L2260