buildgcc: Indicate CXXFLAGS for binutils

CXXFLAGS seems to be used a lot and have to be specified independently
from CFLAGS.

Change-Id: Iff4c76e54a46e908299b532fd848165a3dc04d43
Signed-off-by: Nico Huber <nico.huber@secunet.com>
Reviewed-on: https://review.coreboot.org/17937
Tested-by: build bot (Jenkins)
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>
Reviewed-by: Martin Roth <martinroth@google.com>

util/crossgcc/buildgcc

@@ -579,7 +579,10 @@ build_BINUTILS() {
 		--target=${TARGETARCH} --enable-targets=${TARGETARCH}${ADDITIONALTARGET} \
 		--disable-werror --disable-nls --enable-lto --enable-gold \
 		--enable-interwork --enable-multilib \
-		--enable-plugins --enable-multilibs CFLAGS="$HOSTCFLAGS" || touch .failed
+		--enable-plugins --enable-multilibs
+		CFLAGS="$HOSTCFLAGS" \
+		CXXFLAGS="$HOSTCFLAGS" \
+		|| touch .failed
 	$MAKE $JOBS || touch .failed
 	$MAKE install DESTDIR=$DESTDIR || touch .failed
 }