[libc] Add barebones dl_iterate_phdr implementation (#194196)
Add a basic dl_iterate_phdr implementation so that we can get libunwind
building. This implementation is bare and not fully compliant with the
man page for fully static binaries (which are all that we support
currently with the lack of a dynamic linker) due to the lack of TLS
info, but that can be added at a future date if it is needed, as it is
not needed by libunwind.
Add some very basic smoke tests.