[compiler-rt] Add SANITIZER_CXX_ABI_LIBNAME=libc++ option.

Summary: This is to support Android where libc++abi is part of libc++.

Reviewers: srhines, EricWF

Subscribers: dberris, mgorny, llvm-commits

Differential Revision: https://reviews.llvm.org/D36640

llvm-svn: 310769