selftests/nolibc: use correct clang target for s390/systemz

The target names between GCC and clang differ for s390.
While GCC uses "s390", clang uses "systemz".
This mapping is not handled by tools/scripts/Makefile.include,
so do it in the nolibc-test Makefile.

Acked-by: Willy Tarreau <w@1wt.eu>
Reviewed-by: Shuah Khan <skhan@linuxfoundation.org>
Link: https://lore.kernel.org/r/20240807-nolibc-llvm-v2-14-c20f2f5fc7c2@weissschuh.net
Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>

tools/testing/selftests/nolibc/Makefile

@@ -167,6 +167,9 @@ endif
 # Modify CFLAGS based on LLVM=
 include $(srctree)/tools/scripts/Makefile.include
 
+# GCC uses "s390", clang "systemz"
+CLANG_CROSS_FLAGS := $(subst --target=s390-linux,--target=systemz-linux,$(CLANG_CROSS_FLAGS))
+
 REPORT  ?= awk '/\[OK\][\r]*$$/{p++} /\[FAIL\][\r]*$$/{if (!f) printf("\n"); f++; print;} /\[SKIPPED\][\r]*$$/{s++} \
 		END{ printf("\n%3d test(s): %3d passed, %3d skipped, %3d failed => status: ", p+s+f, p, s, f); \
 		if (f || !p) printf("failure\n"); else if (s) printf("warning\n"); else printf("success\n");; \