Create a whitelist for what we export from libc++_shared.so.

This adds an export_symbols.txt which is a trimmed down list
of symbols that we want to export from libc++ and libc++abi, and
makes use of it during linking to remove anything else.

Change-Id: Ie3b39e88160dd7b4973765c73fea8c75a74b1767
Merge pull request #110 from awong-dev/symbols

Create a whitelist for what we export from libc++_shared.so.