s390: add options to change branch prediction behaviour for the kernel

s390: add options to change branch prediction behaviour for the kernel

Add the PPA instruction to the system entry and exit path to switch
the kernel to a different branch prediction behaviour. The instructions
are added via CPU alternatives and can be disabled with the "nospec"
or the "nobp=0" kernel parameter. If the default behaviour selected
with CONFIG_KERNEL_NOBP is set to "n" then the "nobp=1" parameter can be
used to enable the changed kernel branch prediction.

Acked-by: Cornelia Huck <[email protected]>
Signed-off-by: Martin Schwidefsky <[email protected]>