x86/spec-ctrl: Fold DO_SPEC_CTRL_EXIT_TO_XEN into it's single user

x86/spec-ctrl: Fold DO_SPEC_CTRL_EXIT_TO_XEN into it's single user

With the SPEC_CTRL_EXIT_TO_XEN{,_IST} confusion fixed, it's now obvious that
there's only a single EXIT_TO_XEN path.  Fold DO_SPEC_CTRL_EXIT_TO_XEN into
SPEC_CTRL_EXIT_TO_XEN to simplify further fixes.

When merging labels, switch the name to .L\@_skip_sc_msr as "skip" on its own
is going to be too generic shortly.

No functional change.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
(cherry picked from commit 694bb0f280fd08a4377e36e32b84b5062def4de2)