x86/traps: Rework write_stub_trampoline() to not hardcode the jmp

x86/traps: Rework write_stub_trampoline() to not hardcode the jmp

For CET-IBT, we will need to optionally insert an endbr64 instruction at the
start of the stub.  Don't hardcode the jmp displacement assuming that it
starts at byte 24 of the stub.

Also add extra comments describing what is going on.  The mix of %rax and %rsp
is far from trivial to follow.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
(cherry picked from commit 809beac3e7fdfd20000386453c64a1e2a3d93075)