Including every OS' own built-in byte swapping functions is kind of
undesirable, since it adds yet another build path to ensure compilation
succeeds on.

Given we only support clang, GCC, and MSVC for the time being, we can
utilize their built-in functions directly instead of going through the
OS's API functions.

This shrinks the overall code down to just

if (msvc)
  use msvc's functions
else if (clang or gcc)
  use clang/gcc's builtins
else
  use the slow path