[libunwind] Factor out sigreturn check condition. NFC

[libunwind] Factor out sigreturn check condition. NFC

Create a macro for this instead of duplicating the architecture checks
everywhere. (It's a little redundant to use it when we're checking for a
specific architecture, but I'm also applying it there for consistency.)

Reviewed By: rprichard, MaskRay, #libunwind

Differential Revision: https://reviews.llvm.org/D126342