util/xcompile: Add gnatbind tool

Change-Id: I79c94a1a951fe7e3493b839364a79fa2edb57ff3
Signed-off-by: Nico Huber <nico.h@gmx.de>
Reviewed-on: https://review.coreboot.org/13043
Tested-by: build bot (Jenkins)
Reviewed-by: Patrick Georgi <pgeorgi@google.com>

util/xcompile/xcompile

@@ -268,6 +268,7 @@ OBJDUMP_${TARCH}:=${GCCPREFIX}objdump
 READELF_${TARCH}:=${GCCPREFIX}readelf
 STRIP_${TARCH}:=${GCCPREFIX}strip
 AR_${TARCH}:=${GCCPREFIX}ar
+GNATBIND_${TARCH}:=${GCCPREFIX}gnatbind
 CROSS_COMPILE_${TARCH}:=${GCCPREFIX}