s390/nospec: generate single register thunks if possible

Currently assembler generated expoline thunks are always in a form
__s390_indirect_jump_rXuse_rX even when exrl instruction is available
and no additional register is utilized.

Generate __s390_indirect_jump_rX versions using a single register if the
kernel is built for z10 or newer machine, which have exrl instruction
available. Thunks generated are identical to the ones generated by the
compiler.

This helps to reduce the number of thunks for newer machines generations.

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>