x86/boot: Map the trampoline as read-only

x86/boot: Map the trampoline as read-only

c/s ec92fcd1d08, which caused the trampoline GDT Access bits to be set,
removed the final writes which occurred between enabling paging and switching
to the high mappings.  There don't plausibly need to be any memory writes in
few instructions is takes to perform this transition.

As a consequence, we can remove the RWX mapping of the trampoline.  It is RX
via its identity mapping below 1M, and RW via the directmap.

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