cross: export CC family from BUILD_* for cross compilers

cross: export CC family from BUILD_* for cross compilers

cross-compilers are native recipes that need to be compiled by the host's
compiler. However they do not use native.bbclass

As a consequence, the various CC, CXX etc environment variables are not
correctly set and they will not honor the host compiler name provided
by the BUILD_* variables.

(From OE-Core rev: 110cc22614f92e848bbba9ecaa5e6f089f9a2749)

Signed-off-by: Jérémy Rosen <jeremy.rosen@smile.fr>
Signed-off-by: Ross Burton <ross.burton@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>