xcompile: Store XGCCPATH

It can be useful to pass along to external projects, e.g. payloads.

Change-Id: I61c7bb162e2737a562cbef08b32ebbafd9cf1cb0
Signed-off-by: Nico Huber <nico.h@gmx.de>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/34555
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Patrick Georgi <pgeorgi@google.com>
Reviewed-by: Martin Roth <martinroth@google.com>

util/xcompile/xcompile

@@ -98,6 +98,7 @@ fi
 
 cat <<EOF
 # platform agnostic and host tools
+XGCCPATH:=${XGCCPATH}
 IASL:=${IASL}
 HOSTCC?=${HOSTCC}
 CPUS?=${CORES}