util: Add SPDX license headers to Makefiles

Signed-off-by: Martin Roth <gaumless@gmail.com>
Change-Id: I7cf35132df0bc23f7b6f78014ddd72d58ea2ab8a
Reviewed-on: https://review.coreboot.org/c/coreboot/+/68983
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Matt DeVillier <matt.devillier@amd.corp-partner.google.com>
Reviewed-by: Elyes Haouas <ehaouas@noos.fr>
Reviewed-by: Eric Lai <eric_lai@quanta.corp-partner.google.com>

util/crossgcc/Makefile

@@ -1,3 +1,4 @@
+## SPDX-License-Identifier: GPL-2.0-only
 # if no architecture is specified, set a default
 BUILD_PLATFORM ?= i386-elf
 DEST ?= $(CURDIR)/xgcc