Revert "Move some tests to libc++."

The build system is still linking both libgcc and libcompiler-rt,
which is causing duplicate symbol errors on some architectures. This
requires a fix in the build system, so reverting for now.

This reverts commit e3cccbfd4776644c7cb705d52f51ca948df34f9a.
Merge "Revert "Move some tests to libc++.""