[AArch64] Change AArch64 Windows EH UnwindHelp object to be a fixed object

The UnwindHelp object is used during exception handling by runtime
code. It must be findable from a fixed offset from FP.

This change allocates the UnwindHelp object as a fixed object (as is
done for x86_64) to ensure that both the generated code and runtime
agree on the location of the object.

Fixes https://bugs.llvm.org/show_bug.cgi?id=45346

Differential Revision: https://reviews.llvm.org/D77016

(cherry picked from commit 494abe13)