x86/xpti: don't map stack guard pages

x86/xpti: don't map stack guard pages

Other than for the main mappings, don't even do this in release builds,
as there are no huge page shattering concerns here.

Note that since we don't run on the restructed page tables while HVM
guests execute, the non-present mappings won't trigger the triple fault
issue AMD SVM is susceptible to with our current placement of STGI vs
TR loading.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Andrew Cooper <andrew.cooper3@citrix.com>