Try to fix the issue that doesn't permit us to merge libcxx with libcxxabi

[Smjert]

LLVM CMake has an option when building libc++abi to merge that together with libc++, so that then when you use the toolchain to compile something it's not necessary to specify to link against libc++abi. The option is LIBCXX_ENABLE_STATIC_ABI_LIBRARY.

Last time I tried it failed on the python script that is in charge to merge the two archives. I don't have with me the error though (sorry).

This is low priority and just a nice to have.