tools/nolibc: always use 64-bit mode for s390 header checks

tools/nolibc: always use 64-bit mode for s390 header checks

32-bit s390 support was recently removed from nolibc.
If the compiler defaults to 32-bit during the header checks, they fail.

Make sure to always use 64-bit mode for s390 heafer checks.

Fixes: 169ebcbb9082 ("tools: Remove s390 compat support")
Acked-by: Willy Tarreau <w@1wt.eu>
Acked-by: Heiko Carstens <hca@linux.ibm.com>
Link: https://patch.msgid.link/20251203-nolibc-headers-check-s390-v1-1-5d35e52a83ba@weissschuh.net
Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>