Don't bake all of CC/CPP/CXX into CFLAGS

Capture executable names for CC, CPP, CXX (assumed to be the
first non-CCACHE_BIN word).

This change strips out all of the cross-compiler arguments, (-target,
-B, etc), added to ${CC}, etc via ${CROSSENV} in Makefile.inc1, so it
doesn't infect the build and subsequently the test.

Add comments noting why this logic is being added, and why the logic in
r305041 was necessary/what it was trying to achieve.

This is required after recent changes made to the toolchain to always
specify --sysroot, -target, -B, etc with clang in buildworld (presumably
r304681).

Reviewed by: rodrigc (earlier version)
Reported by: Jenkins (FreeBSD_HEAD job from 559+)
MFC after: 12 days
X-MFC with: r304681, r305041
Sponsored by: EMC / Isilon Storage Division
Differential Revision: https://reviews.freebsd.org/D7732