[libc] Add a __libc_extensions_fini hook

Some things that initialize structures in __libc_extensions_init want
to clean up after themselves at program shutdown time. Currently they
are doing this via exit handlers. This cleanup then ends up
interleaved with other exit time handlers that depend on
__libc_extensions_init-ed functionality.

This adds a hook for those extensions that runs after C exit handlers
have run.

MA-368 #progress

Change-Id: Ic13b44ad7bc51fcd5da4975009d70d839b98c686

[musl] Add implementation of __cxa_thread_atexit_impl

C++ ABI expects this function to be provided by the C library, libcxxabi
it has its own implementation that is used in case when C library
doesn't provide one, but that implementation has number of limitations
and pulls in additional dependencies which are undesirable
(__dso_handle).

This change adds implementation of __cxa_thread_atexit_impl into our C
library. This version is resembles the implementation __cxa_atexit, and
is significantly simpler than implementations of
__cxa_thread_atexit_impl in other C libraries because our C library
doesn't support dlclose.

MG-655

Change-Id: I1f79853f9cff3b8ecf9c35419f74bf3463f3df58

[ulib][musl] Reduce unnecessary exports

There are still lots of symbols in the export list that should not
really be part of the ABI. But this picks off some low-hanging fruit.

Change-Id: I86168858de89bd327fb70cd4ca639caa1e502702

[magenta] Initial commit