Promote nameless lambda used by dl_iterate_phdr to named function.

Summary:
This cleans up control flow inside findUnwindSections, and will make
it easier to replace this code in a following patch. Also, expose the
data structure to allow use by a future replacment function.

Reviewers: mstorsjo, miyuki

Subscribers: krytarowski, libcxx-commits

Tags: #libc

Differential Revision: https://reviews.llvm.org/D75637