[libcxxabi] Refactor CMakeLists.txt's handling of compile and link flags to suppress warnings.

Summary:
This patch mirrors the recent change to libc++ found here http://reviews.llvm.org/D6277.
This fixes PR20395 (http://llvm.org/bugs/show_bug.cgi?id=20395).

Reviewers: jroelofs, mclow.lists, danalbert

Reviewed By: danalbert

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D6286

git-svn-id: https://llvm.org/svn/llvm-project/libcxxabi/trunk@222252 91177308-0d34-0410-b5e6-96231b3b80d8