[libcxxabi] Document the standalone build's requirements on llvm-config and llvm sources.

The current documentation does not explain that the standalone build requires
the LLVM sources. This patch updates the documentation to reflect this
requirement and explains how to manually specify the location of the required
files.


git-svn-id: https://llvm.org/svn/llvm-project/libcxxabi/trunk@220049 91177308-0d34-0410-b5e6-96231b3b80d8
Add an .arcconfig for libc++abi.


git-svn-id: https://llvm.org/svn/llvm-project/libcxxabi/trunk@220050 91177308-0d34-0410-b5e6-96231b3b80d8