Split findUnwindSectionsByPhdr into target-specific functions.

Summary:
This further cleans up the control flow and makes it easier to
optimize and replace portions in a subsequent patch.

This should be NFC, but given the amount of #ifdeffing here,
it may not be. So will watch the buildbots closely.

Also, as this is purely moving existing code around, I plan to
ignore the lint errors.

Reviewers: compnerd, miyuki, mstorsjo

Subscribers: libcxx-commits

Tags: #libc

Differential Revision: https://reviews.llvm.org/D75705