[libcxxabi] Rework CMakeLists.txt into modules

Refactor cmake to remove dependence on LLVM's cmake modules.
This improves handling of cmake checks when cross compiling and brings
libcxxabi in line with libcxx and other project modules.

Differential revision: https://reviews.llvm.org/D33635

git-svn-id: https://llvm.org/svn/llvm-project/libcxxabi/trunk@304374 91177308-0d34-0410-b5e6-96231b3b80d8
5 files changed