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>