s390/ftrace: use la instead of aghik in return_to_handler()

Nathan Chancellor reported the following build error when compiling the
kernel with CONFIG_MARCH_Z10=y:

  arch/s390/kernel/mcount.S: Assembler messages:
  arch/s390/kernel/mcount.S:140: Error: Unrecognized opcode: `aghik'

The aghik instruction is only available since z196. Use the la instruction
instead which is available for all machines.

Reported-by: Nathan Chancellor <nathan@kernel.org>
Closes: https://lore.kernel.org/all/20230725211105.GA224840@dev-arch.thelio-3990X


Fixes: 1256e70a ("s390/ftrace: enable HAVE_FUNCTION_GRAPH_RETVAL")
Reviewed-by: Sven Schnelle <svens@linux.ibm.com>
Tested-by: Nathan Chancellor <nathan@kernel.org> # build
Link: https://lore.kernel.org/r/20230726061834.1300984-1-hca@linux.ibm.com


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