s390/boot: Introduce jump_to_kernel() function

Introduce a global function that jumps from the decompressor to the
decompressed kernel. Put its address into svc_old_psw, from where GDB
can take it without loading decompressor symbols. It should be
available throughout the entire decompressor execution, because it's
placed there statically, and nothing in the decompressor uses the SVC
instruction.

Acked-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
Tested-by: Alexander Gordeev <agordeev@linux.ibm.com>
Link: https://lore.kernel.org/r/20250625154220.75300-2-iii@linux.ibm.com


Signed-off-by: Alexander Gordeev <agordeev@linux.ibm.com>