add riscv64 support

[cybaol]

Use simde package to support riscv64
Closes #93

[wszqkzqk]

See: https://github.com/simd-everywhere/simde

If you define SIMDE_ENABLE_NATIVE_ALIASES before including SIMDe you can use the same names as the native functions. Unfortunately, this is somewhat error-prone due to portability issues in the APIs, so it's recommended to only do this for testing. When SIMDE_ENABLE_NATIVE_ALIASES is undefined only the versions prefixed with simde_ will be available; for example, the MMX _mm_add_pi8 intrinsic becomes simde_mm_add_pi8, and __m64 becomes simde__m64.

Perhaps a more appropriate approach would be to use simde's functions instead of x86's. 🤔Then it may be possible to use simde for all platforms to provide the same behavior.

... it is important to note that you do not need two separate versions (one using SIMDe, the other native). If the native functions are available SIMDe will use them, and compilers easily optimize away any overhead from SIMDe; all they have to do is some basic inlining.