s390: always use the packed stack layout

s390: always use the packed stack layout

-mpacked-stack option has been supported by both minimum
gcc and clang versions for a while. With commit e2bc3e91d91e
("scripts/min-tool-version.sh: Raise minimum clang version to 13.0.0
for s390") minimum clang version now also supports a combination
of flags -mpacked-stack -mbackchain -pg -mfentry and fulfills
all requirements to always enable the packed stack layout.

Reviewed-by: Heiko Carstens <[email protected]>
Signed-off-by: Vasily Gorbik <[email protected]>