[libcxxabi] Allow target flags to affect configuration tests.

Summary:
This patch changes the libc++abi CMake so that it adds certain target flags like  '-m32' or '--gcc-toolchain' before including `config-ix.cmake`.
Since these flags can affect things like `check_library_exists([...])` they needed to be added before the tests are performed.

Additionally this patch adds `LIBCXXABI_BUILD_32_BITS` which defaults to `LLVM_BUILD_32_BITS`.

This patch fixes:

https://llvm.org/bugs/show_bug.cgi?id=27950
https://llvm.org/bugs/show_bug.cgi?id=27959

Reviewers: danalbert, jroelofs, bcraig, compnerd, EricWF

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D20886

llvm-svn: 271470