[ARM] don't emit constant pool for Thumb1 XO/stack guard combo

Currently for armv6-m and armv8-m.baseline, we emit constant pool code when we
use execute-only (XO) in combination with stack guards.

XO is a new feature for armv6-m, and this patch is part of a series of patches
that substitutes constant pool generation with the tMOVi32imm equivalent.

However XO for armv8-m.baseline has been available for about 6 years, and so
for armv8-m.baseline this is a bugfix.

Reviewed By: simonwallis2, olista01

Differential Revision: https://reviews.llvm.org/D155170