v5.18 backports for s390 expolines

Bugzilla: https://bugzilla.redhat.com/show_bug.cgi?id=2072713
Bugzilla: https://bugzilla.redhat.com/show_bug.cgi?id=2121735
Tested: Passed beaker CKI tests. https://beaker.engineering.redhat.com/jobs/7231265

This MR adds backports for s390 expolines. These commits were included.

• s390: replace cc-option-yn uses with cc-option
• s390/entry: remove unused expoline thunk
• s390: remove unused expoline to BC instructions
• s390/nospec: generate single register thunks if possible
• s390/nospec: add an option to use thunk-extern
• s390/nospec: align and size extern thunks
• s390/nospec: build expoline.o for modules_prepare target
• s390/nospec: remove unneeded header includes

Signed-off-by: Julia Denham jdenham@redhat.com