Add a way to apply CFLAGS only when building the given architecture. This
is useful primarily on a system used for cross-building, when you have a
set of flags to apply to the TARGET_ARCH being cross-built but don't want
those settings applied to building the cross-tools or other components that
run on the build host machine.
Support CXXFLAGS.${MACHINE_ARCH} as well as CFLAGS. This allows different
C++ options for toolchain versus target when cross-building.
0adc7b49f4