Make [u8]::reverse() 5x faster

Make [u8]::reverse() 5x faster

Since LLVM doesn't vectorize the loop for us, do unaligned reads
of a larger type and use LLVM's bswap intrinsic to do the
reversing of the actual bytes.  cfg!-restricted to x86 and
x86_64, as I assume it wouldn't help on things like ARMv5.

Also makes [u16]::reverse() a more modest 1.5x faster by
loading/storing u32 and swapping the u16s with ROT16.

Thank you ptr::*_unaligned for making this easy :)