[libcxxabi] Teach CMake better ways to find the libc++ source directory (and misc...

[libcxxabi] Teach CMake better ways to find the libc++ source directory (and misc cleanup).

Summary:
The main section of this patch teaches CMake  a new option `LIBCXXABI_LIBCXX_PATH` that specifies the path to the libcxx source root. This information is passed to lit so that it can better find libc++'s python module. `LIBCXXABI_LIBCXX_PATH` is also used to help find the libc++ headers.

The rest of this patch is misc cleanup, mostly to make pep8 and pylint happy.
I've also copied libc++'s .gitignore into libc++abi.

Reviewers: jroelofs, danalbert

Reviewed By: danalbert

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D7130

llvm-svn: 226855