[CMake][libcxx] Fix the --target and --gcc-toolchain flag handling

CMake has the problem with the single dash variant because of the
space, so use the double dash with equal sign version. We also
don't have to pass the target triple when checking for compiler-rt
since that flag is already included in compile flags now.

Differential Revision: https://reviews.llvm.org/D32068

llvm-svn: 300409