buildgcc: enable multilib for gcc

Make the gcc build system create multiple libgcc.a instances for
different ABIs.

Change-Id: I1c888bf751bf43566da8927ed0aedb53857363bf
Signed-off-by: Patrick Georgi <pgeorgi@chromium.org>
Reviewed-on: https://review.coreboot.org/13625
Tested-by: build bot (Jenkins)
Reviewed-by: Stefan Reinauer <stefan.reinauer@coreboot.org>

util/crossgcc/buildgcc

@@ -439,6 +439,7 @@ build_GCC() {
 		--disable-libssp --disable-bootstrap --disable-nls \
 		--disable-libquadmath --without-headers \
 		--disable-threads \
+		--enable-interwork --enable-multilib \
 		--disable-libatomic --disable-libcc1 --disable-decimal-float \
 		${GCC_OPTIONS} --enable-languages="${LANGUAGES}" \
 		--with-gmp=$DESTDIR$TARGETDIR --with-mpfr=$DESTDIR$TARGETDIR \