get rid of llvm_intrinsics.zig

[travisstaloch]

convert all of the hacks found here and here to zig builtin calls. this will hopefully make it possible to build on mac and other platforms and close #1.

1. try to use existing zig intrinsics if possible
2. work toward implementing them in the zig compiler

• [x] saturating arithmetic - https://github.com/ziglang/zig/pull/9619
• [ ] carrylessMu()l (llvm.x86.pclmulqdq)
• [ ] shuffleEpi8() (llvm.x86.avx2.pshuf.b)
• [ ] shuffleEpi32() (llvm.x86.ssse3.pshuf.b.128)
• [ ] vpalignr() (llvm.x86.vpalignr)
• [ ] _mm256_movemask_epi8() (llvm.x86.avx2.pmovmskb)
• [ ] _mm_maddubs_epi16() (llvm.x86.ssse3.pmadd.ub.sw.128)
• [ ] _mm_madd_epi16() (llvm.x86.sse2.pmadd.wd)
• [ ] _mm_packus_epi32() (llvm.x86.sse41.packusdw)
• [x] _prevN()
  • converted to @shuffle instruction in aarch64 branch