libcxxabi cmake: Use HandleLLVMOptions.cmake, don't manually add -std=c++11.

No intended behavior change on Linux and Mac OS X.  On Windows, having libcxxabi
in one's checkout should now no longer break running cmake. (cl.exe supports
c++11, but doesn't understand a -std= flag.)


git-svn-id: https://llvm.org/svn/llvm-project/libcxxabi/trunk@213214 91177308-0d34-0410-b5e6-96231b3b80d8