s390/syscalls: Convert filechk to if_changed

s390/syscalls: Convert filechk to if_changed

The filechk macro always executes the syscalltbl script (and discards
the output if there are no changes).

Using if_changed is more efficient because it avoids running the script
when the target is up-to-date and the command remains unchanged.

All other architectures use if_changed for generating syscall headers.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Link: https://lore.kernel.org/r/20241111134603.2063226-3-masahiroy@kernel.org
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>