compiler.h: unify system ifdef cruft here

Shove a lot of the HOSTCC and related #ifdef checking crap into the new
compiler.h header so that we can keep all other headers nice and clean.

Also introduce custom uswap functions so we don't have to rely on the non
standard implementations that a host may (or may not in the case of OS X)
provide.  This allows mkimage to finally build cleanly on an OS X system.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>