Use as much of the macOS endian support as we can.

Turns out that although there's no <endian.h> or <sys/endian.h>, there
are <machine/endian.h> and <sys/_endian.h>, and they're included by
other system headers such as <dirent.h>.

Reuse the contents of <sys/_endian.h> here for better interop.

Bug: http://b/139203733
Test: treehugger
Change-Id: Ic0e9bfa1a5b56d05e9e542839d237b6ceae4aa8c