Log Message:
lang/gcc*: Set CC_FOR_BUILD and CXX_FOR_BUILD if cross-compiling.

Verified gcc6 and gcc7 cross-compile on NetBSD, at least if the
gcc-fortran option is disabled.  gcc8 and gcc9 have trouble because
of python (why do these have a full dependency on python?); gcc10 and
gcc12 are missing gcc/config/mips/netbsd64.h; gcc12 and gcc13 have a
cross-compile bug with missing __LIBGCC_*__ macros -- but in any
case, setting CC_FOR_BUILD and CXX_FOR_BUILD is obviously necessary
for cross-compilation.  Conditional on USE_CROSS_COMPILE, so no harm
to native builds.