s390/entry: Scrub r12 register on kernel entry

In the Linux kernel, the following vulnerability has been resolved:

s390/entry: Scrub r12 register on kernel entry

Before commit f33f2d4c7c80 ("s390/bp: remove TIF_ISOLATE_BP"),

all entry handlers loaded r12 with the current task pointer

(lg %r12,__LC_CURRENT) for use by the BPENTER/BPEXIT macros. That

commit removed TIF_ISOLATE_BP, dropping both the branch prediction

macros and the r12 load, but did not add r12 to the register clearing

sequence.

Add the missing xgr %r12,%r12 to make the register scrub consistent

across all entry points.