Translate `builtin_assume` and unaligned SIMD intrinsics

[thedataking]

Unaligned vector types such as __m128_u or __256_u not not have any equivalents in Rust so we translate them to their aligned equivalents. This is unsound so we emit warning that the program may be mistranspiled. These fixes are required to transpile the zstd compressor on modern x86 hosts.