This re-applies commit r260235. However, this time we add -gcc-toolchain to the compiler's flags when the user has specified the LIBCXX_GCC_TOOLCHAIN variable. git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@260515 91177308-0d34-0410-b5e6-96231b3b80d8