s390/speculation: Use statically initialized const for instructions

s390/speculation: Use statically initialized const for instructions

In preparation for FORTIFY_SOURCE performing compile-time and run-time
field bounds checking for memcpy(), memmove(), and memset(), avoid
confusing the checks when using a static const source.

Move the static const array into a variable so the compiler can perform
appropriate bounds checking.

Signed-off-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20210616201823.1245603-1-keescook@chromium.org
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>