[WebAssembly] Support -stdlib=libc++ switch

Referenced implementation from Fuchsia and Darwin Toolchain.
Still only support CST_Libcxx.  Now checks that the argument
is really '-stdlib=libc++', and display error.

Also, now will pass -lc++ and -lc++abi to the linker.

Patch by Patrick Cheng!

Differential Revision: https://reviews.llvm.org/D41937

llvm-svn: 322382