This allows us to specify cross compilers for our guests. This is
useful for building test images/programs. Currently we re-run the
compile test for each target. I couldn't think of a way to cache the
value for a given arch without getting messier configure code.

The cross compiler for the guest is visible to each target as
CROSS_CC_GUEST in config-target.mak. This is quoted to handle the case
of --cc="ccache gcc".
Signed-off-by: NAlex Bennée <alex.bennee@linaro.org>
Reviewed-by: NRichard Henderson <richard.henderson@linaro.org>