gdb: fix cross-build for aarch64.

Usually, this autoconf override is set while do_configure, but gdb
also runs configure scripts during do_build; override again.

srcpkgs/gdb/template

@@ -34,6 +34,9 @@ vopt_conflict gdbserver static
 post_extract() {
 	sed -i 's,sgidefs.h,asm/sgidefs.h,' gdb/mips-linux-nat.c
 }
+pre_build() {
+	export gl_cv_func_gettimeofday_clobber=no
+}
 post_install() {
 	# resolve conflicts with binutils
 	rm -rf ${DESTDIR}/usr/include ${DESTDIR}/usr/lib