s390x: Add a linker script to assembly snippets

A linker script has a few benefits:
- Random data doesn't end up in the binary breaking tests
- We can easily define a lowcore and load the snippet from 0x0 instead
of 0x4000 which makes asm snippets behave like c snippets
- We can easily define an invalid PGM new PSW to ensure an exit on a
guest PGM

Signed-off-by: Janosch Frank <frankja@linux.ibm.com>
Reviewed-by: Claudio Imbrenda <imbrenda@linux.ibm.com>
Link: https://lore.kernel.org/r/20230112154548.163021-4-frankja@linux.ibm.com
Message-Id: <20230112154548.163021-4-frankja@linux.ibm.com>