x86: arrange for ENDBR zapping from <vendor>_ctxt_switch_masking()

x86: arrange for ENDBR zapping from <vendor>_ctxt_switch_masking()

While altcall is already used for them, the functions want announcing in
.init.rodata.cf_clobber, even if the resulting static variables aren't
otherwise used.

While doing this also move ctxt_switch_masking to .data.ro_after_init.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>
(cherry picked from commit 044168fa3a65b6542bda5c21e373742de1bd5980)