[libunwind] Make findUnwindSectionsByPhdr static

Currently, this function is present in the dynsym table of
libunwind.so (on ELF targets). Make the function static instead.

In the previous release (LLVM 10.x), this function was instead a lambda
function inside LocalAddressSpace::findUnwindSections, and because
LocalAddressSpace was marked with _LIBUNWIND_HIDDEN, the lambda
function was also a hidden symbol.

Differential Revision: https://reviews.llvm.org/D86372