[libcxx] Allow target flags to affect CMake configuration tests

Summary:
This patch changes the libc++ 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.

This patch fixes:

https://llvm.org/bugs/show_bug.cgi?id=24322

Reviewers: danalbert, jroelofs, bcraig, compnerd

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D20887

llvm-svn: 271460
Cr-Mirrored-From: sso://chromium.googlesource.com/_direct/external/github.com/llvm/llvm-project
Cr-Mirrored-Commit: a4f2460fa54307d00c099802460e96ccd7db779b
2 files changed