selftests/nolibc: be more specific about variables affecting nolibc-test

Only one of these variables is used.
$CC is preferred over $CROSS_COMPILE.

Make this clear in the help message.

Suggested-by: Willy Tarreau <w@1wt.eu>
Link: https://lore.kernel.org/lkml/20250817093905.GA14213@1wt.eu/
Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>

Author: t-8ch

tools/testing/selftests/nolibc/Makefile.nolibc

@@ -267,7 +267,7 @@ help:
 	@echo "  all               call the \"run\" target below"
 	@echo "  help              this help"
 	@echo "  sysroot           create the nolibc sysroot here (uses \$$ARCH)"
-	@echo "  nolibc-test       build the executable (uses \$$CC and \$$CROSS_COMPILE)"
+	@echo "  nolibc-test       build the executable (uses \$$CC or \$$CROSS_COMPILE)"
 	@echo "  libc-test         build an executable using the compiler's default libc instead"
 	@echo "  run-user          runs the executable under QEMU (uses \$$XARCH, \$$TEST)"
 	@echo "  initramfs.cpio    prepare the initramfs archive with nolibc-test"