s390/nospec: rename assembler generated expoline thunks

The assembler version of the expoline thunk use the naming
__s390x_indirect_jump_rxuse_ry while the compiler generates names
like __s390_indirect_jump_rx_use_ry. Make the naming more consistent.

Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>