[compiler-rt] Cleanup usage of C++ ABI library

Add missed value "libcxxabi" and introduce SANITIZER_TEST_CXX for linking
unit tests. This needs to be a full C++ library and cannot be libcxxabi.

Recommit r354132 which I reverted in r354153 because it broke a sanitizer
bot. This was because of the "fixes" for pthread linking, so I've removed
these changes.

Differential Revision: https://reviews.llvm.org/D58012

llvm-svn: 354198