Remove the various build flag hacks for GCC cross-compile.

The xtoolchain GCC packages have not required these flags since ports
commits r465416 and r466701. The in-tree GCC 4.2.1 has also been patched
in r335716 and r335717 to correctly honor --sysroot when looking for
includes and libraries.

Reviewed by: bdrewery
Sponsored by: DARPA / AFRL
Differential Revision: https://reviews.freebsd.org/D16055