util/abuild: Ensure .xcompile exists

abuild requires the `.xcompile` file to be present already before it
runs any actual `make` builds that would generate it.

Change-Id: Ib485e7741b7700fa241c192e60900ae5f1d977f5
Signed-off-by: Nico Huber <nico.h@gmx.de>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/29483
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Angel Pons <th3fanbus@gmail.com>
Reviewed-by: Patrick Georgi <pgeorgi@google.com>

util/abuild/abuild

@@ -770,6 +770,8 @@ if ! mkdir -p "$TARGET"; then
 	exit 1
 fi
 
+$MAKE -C"${ROOT}" .xcompile || exit 1
+
 customizing=$(echo "$customizing" | cut -c3-)
 if [ "$customizing" = "" ]; then
 	customizing="default configuration"