Add RISC-V unaligned access

[guoyuqi020]

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:

zstd/lib/common/xxhash.h

Lines 2254 to 2260 in bfd8ad8

	#ifndef XXH_FORCE_ALIGN_CHECK /* can be defined externally */
	/* don't check on sizeopt, x86, aarch64, or arm when unaligned access is available */
	# if XXH_SIZE_OPT >= 1 || \
	defined(__i386) || defined(__x86_64__) || defined(__aarch64__) || defined(__ARM_FEATURE_UNALIGNED) \
	|| defined(_M_IX86) || defined(_M_X64) || defined(_M_ARM64) || defined(_M_ARM) /* visual */
	# define XXH_FORCE_ALIGN_CHECK 0
	# else