util/crossgcc: Tell gcc that it'll use gnu as and ld

Otherwise it reduces its expectations on what as and ld take in terms
of arguments, which breaks some edk2 related builds because tons of
-I$path_to_stuff arguments aren't passed along.

Change-Id: I53f87442de03d5ead8a6632d3102d5502065b828
Signed-off-by: Patrick Georgi <pgeorgi@google.com>
Reviewed-on: https://review.coreboot.org/28534
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>
Reviewed-by: Martin Roth <martinroth@google.com>

util/crossgcc/buildgcc

@@ -787,6 +787,7 @@ build_cross_GCC() {
 		--with-system-zlib \
 		--with-gmp="$DESTDIR$TARGETDIR" --with-mpfr="$DESTDIR$TARGETDIR" \
 		--with-mpc="$DESTDIR$TARGETDIR" \
+		--with-gnu-as --with-gnu-ld \
 		--with-pkgversion="coreboot toolchain v$CROSSGCC_VERSION $CROSSGCC_DATE" \
 		&& \
 	mkdir -p gcc/$TARGETARCH && \