s390/kasan: provide uninstrumented __strlen

s390/kasan: provide uninstrumented __strlen

s390 kasan code uses sclp_early_printk to report initialization
failures. The code doing that should not be instrumented, because kasan
shadow memory has not been set up yet. Even though sclp_early_core.c is
compiled with instrumentation disabled it uses strlen function, which
is instrumented and would produce shadow memory access if used. To
avoid that, introduce uninstrumented __strlen function to be used
instead.

Before commit c5f0ca38846e ("s390/kasan: improve string/memory functions
checks") few string functions (including strlen) were escaping kasan
instrumentation due to usage of platform specific versions which are
implemented in inline assembly.

Fixes: c5f0ca38846e ("s390/kasan: improve string/memory functions checks")
Acked-by: Ilya Leoshkevich <iii@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>