x86/syscalls/32: Ignore arch_prctl for other architectures

sys_arch_prctl is only provided on x86, and there is no reason
to add it elsewhere. However, including it on the 32-bit syscall
table caused a warning for most configurations on non-x86:

:1328:2: warning: #warning syscall arch_prctl not implemented [-Wcpp]

This adds an exception to the syscall table checking script.

Fixes: 79170fda ("x86/syscalls/32: Wire up arch_prctl on x86-32")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Cc: Kyle Huey <khuey@kylehuey.com>
Link: http://lkml.kernel.org/r/20170323151904.706286-1-arnd@arndb.de


Signed-off-by: Thomas Gleixner <tglx@linutronix.de>