[CMake] Don't install c++abi headers in standalone libc++ build

This is a refinement on r337833. Previously we were installing two
copies of c++abi headers in libc++ build directory, one in
include/c++build and another one in include/c++/v1. However, the
second copy is unnecessary when building libc++ standalone.

Differential Revision: https://reviews.llvm.org/D49752

llvm-svn: 337979