s390/nospec: move to single register thunks

Assembler generated expoline thunks were in a form
__s390_indirect_jump_rXuse_rX when exrl instruction has not been available.

Now with z10 as minimum supported machine generation there
is no need for 2 register thunks, always generate
__s390_indirect_jump_rX versions.

Acked-by: Heiko Carstens <hca@linux.ibm.com>
Acked-by: Sumanth Korikkar <sumanthk@linux.ibm.com>
Acked-by: Ilya Leoshkevich <iii@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>