payloads/external/skiboot/Makefile: fix output on `make clean`

skiboot's Makefile always executes $(CC) to determine whether its clang
or GCC and not setting CROSS for clean target results in this annoying
output (assuming `powerpc64-linux-gcc` isn't available):

    make[2]: powerpc64-linux-gcc: No such file or directory

Change-Id: I242b2d7c1bdf1bbd70fd4e4e0605341fe8301ca5
Signed-off-by: Sergii Dmytruk <sergii.dmytruk@3mdeb.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/67053
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Krystian Hebel <krystian.hebel@3mdeb.com>

payloads/external/skiboot/Makefile

@@ -32,5 +32,6 @@ distclean: clean
 clean:
 	# Redefine RM because it's used like `$(RM) non-existent-file`
 	# Also ignore useless messages about removing test files
-	[ ! -d $(skiboot_dir) ] || $(MAKE) -C $(skiboot_dir) RM="rm -rf" clean > /dev/null
+	[ ! -d $(skiboot_dir) ] || \
+	        $(MAKE) -C $(skiboot_dir) RM="rm -rf" CROSS="$(skiboot_cross)" clean > /dev/null
 	rm -rf $(build_dir)