[libunwind] Remove compatibility support for macOS 10.6

Remove `_dyld_find_unwind_sections` implementation for macOS that is
10.6 or previous. 10.6 is no longer supported for TOT libunwind after
removing its libkeymgr dependency.

Reviewed By: mstorsjo, pete, #libunwind

Differential Revision: https://reviews.llvm.org/D86104