selftests/nolibc: don't pass CC to toplevel Makefile

The toplevel Makefile is capable of calculating CC from CROSS_COMPILE
and/or ARCH.

Stop passing the unnecessary variable.

Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>
Acked-by: Willy Tarreau <w@1wt.eu>
Link: https://lore.kernel.org/r/20250719-nolibc-llvm-system-v1-2-1730216ce171@weissschuh.net

Author: t-8ch

tools/testing/selftests/nolibc/Makefile.nolibc

@@ -263,7 +263,7 @@ REPORT  ?= awk '/\[OK\][\r]*$$/{p++} /\[FAIL\][\r]*$$/{if (!f) printf("\n"); f++
 		printf("\nSee all results in %s\n", ARGV[1]); }'
 
 # Execute the toplevel kernel Makefile
-KBUILD_MAKE = $(MAKE) -C $(srctree) ARCH=$(ARCH) CC=$(CC) CROSS_COMPILE=$(CROSS_COMPILE)
+KBUILD_MAKE = $(MAKE) -C $(srctree) ARCH=$(ARCH) CROSS_COMPILE=$(CROSS_COMPILE)
 
 help:
 	@echo "Supported targets under selftests/nolibc:"