tools/build: Fix s390(x) cross-compilation with clang

The heuristic to derive a clang target triple from a GCC one does not work
for s390. GCC uses "s390-linux" while clang expects "s390x-linux" or
"powerz-linux".

Add an explicit override.

Signed-off-by: Thomas Weißschuh <thomas.weissschuh@linutronix.de>
Link: https://lore.kernel.org/r/20250620-tools-cross-s390-v2-1-ecda886e00e5@linutronix.de
Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>

Author: t-8ch

tools/scripts/Makefile.include

@@ -101,7 +101,9 @@ else ifneq ($(CROSS_COMPILE),)
 # Allow userspace to override CLANG_CROSS_FLAGS to specify their own
 # sysroots and flags or to avoid the GCC call in pure Clang builds.
 ifeq ($(CLANG_CROSS_FLAGS),)
-CLANG_CROSS_FLAGS := --target=$(notdir $(CROSS_COMPILE:%-=%))
+CLANG_TARGET := $(notdir $(CROSS_COMPILE:%-=%))
+CLANG_TARGET := $(subst s390-linux,s390x-linux,$(CLANG_TARGET))
+CLANG_CROSS_FLAGS := --target=$(CLANG_TARGET)
 GCC_TOOLCHAIN_DIR := $(dir $(shell which $(CROSS_COMPILE)gcc 2>/dev/null))
 ifneq ($(GCC_TOOLCHAIN_DIR),)
 CLANG_CROSS_FLAGS += --prefix=$(GCC_TOOLCHAIN_DIR)$(notdir $(CROSS_COMPILE))