s390/nospec: prefer local labels in .set directives

Use local labels in .set directives to avoid potential compile errors
with LTO + clang. See commit 334865b2

 ("x86/extable: Prefer local
labels in .set directives") for further details.

Since s390 doesn't support LTO currently this doesn't fix a real bug
for now, but helps to avoid problems as soon as required pieces have
been added to llvm.

Signed-off-by: Heiko Carstens <hca@linux.ibm.com>