[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

[musl] Use #pragma once in musl's internal headers

This only replaces existing ifdef guards in internal headers. It doesn't
add them to files that didn't have guards, or to public headers.

Change-Id: Ie012ac77ed95ff476291c7391230af086ad8a019

[magenta] Initial commit