x86/entry: Introduce EFRAME_* constants

x86/entry: Introduce EFRAME_* constants

restore_all_guest() does a lot of manipulation of the stack after popping the
GPRs, and uses raw %rsp displacements to do so.  Also, almost all entrypaths
use raw %rsp displacements prior to pushing GPRs.

Provide better mnemonics, to aid readability and reduce the chance of errors
when editing.

No functional change.  The resulting binary is identical.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
(cherry picked from commit 37541208f119a9c552c6c6c3246ea61be0d44035)