Add KMSAN support for the s390 implementations of the string functions. 
Do this similar to how it's already done for KASAN, except that the
optimized memset{16,32,64}() functions need to be disabled: it's important
for KMSAN to know that they initialized something.

The way boot code is built with regard to string functions is problematic,
since most files think it's configured with sanitizers, but boot/string.c
doesn't.  This creates various problems with the memset64() definitions,
depending on whether the code is built with sanitizers or fortify.  This
should probably be streamlined, but in the meantime resolve the issues by
introducing the IN_BOOT_STRING_C macro, similar to the existing
IN_ARCH_STRING_C macro.

Link: https://lkml.kernel.org/r/20240621113706.315500-33-iii@linux.ibm.comSigned-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
Reviewed-by: Alexander Potapenko <glider@google.com>
Acked-by: Heiko Carstens <hca@linux.ibm.com>
Cc: Alexander Gordeev <agordeev@linux.ibm.com>
Cc: Christian Borntraeger <borntraeger@linux.ibm.com>
Cc: Christoph Lameter <cl@linux.com>
Cc: David Rientjes <rientjes@google.com>
Cc: Dmitry Vyukov <dvyukov@google.com>
Cc: Hyeonggon Yoo <42.hyeyoo@gmail.com>
Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Cc: <kasan-dev@googlegroups.com>
Cc: Marco Elver <elver@google.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Masami Hiramatsu (Google) <mhiramat@kernel.org>
Cc: Pekka Enberg <penberg@kernel.org>
Cc: Roman Gushchin <roman.gushchin@linux.dev>
Cc: Steven Rostedt (Google) <rostedt@goodmis.org>
Cc: Sven Schnelle <svens@linux.ibm.com>
Cc: Vasily Gorbik <gor@linux.ibm.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>