s390/ftrace: move hotpatch trampolines to mcount.S

Move the ftrace hotpatch trampolines to mcount.S. This allows to make
use of the standard SYM_CODE macros which again makes sure that the
hotpatch trampolines follow the function alignment rules of the rest
of the kernel.

Signed-off-by: Heiko Carstens <hca@linux.ibm.com>
Acked-by: Ilya Leoshkevich <iii@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>