[libunwind] Allow target flags to affect CMake configuration tests

Summary:
This patch changes the libunwind 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 LIBUNWIND_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: jroelofs, danalbert, bcraig, rmaprath, compnerd

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D20889

llvm-svn: 271458