util/xcompile/xcompile: Define GCOV_${TARCH}

When payloads analyze the coverage using gcov (or lcov), the gcov
version must match the CC version. Otherwise gcov would fail to parse
the .gcno files.

Therefore, define GCOV_${TARCH} in xcompile, so that payloads don't need
to do tedious string manipulations to find the right gcov path.

Change-Id: If2fc329810c463a3d2c56deaf4e4a3fc3c0a3ed9
Signed-off-by: Yu-Ping Wu <yupingso@chromium.org>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/65840
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Julius Werner <jwerner@chromium.org>
Reviewed-by: Patrick Georgi <patrick@coreboot.org>

util/xcompile/xcompile

@@ -285,6 +285,7 @@ cat <<EOF
 CPP_${TARCH}:=${GCCPREFIX}cpp
 AS_${TARCH}:=${GCCPREFIX}as ${ASFLAGS}
 LD_${TARCH}:=${GCCPREFIX}ld${LINKER_SUFFIX} ${LDFLAGS}
+GCOV_${TARCH}:=${GCCPREFIX}gcov
 EOF
 
 	if [ "${TARCH}" = "arm64" ] && \