[S390] maccess: add weak attribute to probe_kernel_write

probe_kernel_write() gets used to write to the kernel address space.
E.g. to patch the kernel (kgdb, ftrace, kprobes...). Some architectures
however enable write protection for the kernel text section, so that
writes to this region would fault.
This patch allows to specify an architecture specific version of
probe_kernel_write() which allows to handle and bypass write protection
of the text segment.
That way it is still possible to catch random writes to kernel text
and explicitly allow writes via this interface.

Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>