x86/entry: Make IDT entrypoints CET-IBT compatible

x86/entry: Make IDT entrypoints CET-IBT compatible

Each IDT vector needs to land on an endbr64 instruction.  This is especially
important for the #CP handler, which will recurse indefinitely if the endbr64
is missing, eventually escalating to #DF if guard pages are active.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
(cherry picked from commit e702e36d1d519f4b66086650c1c47d6bac96d4b9)

Also include the continue_pv_domain() change from c/s 954bb07fdb5fad which is
also in entry.S