Let cloudabi-toolchain use LLVM 5.0.

Now that we have a devel/llvm50 port, let's switch the CloudABI
toolchain port from LLVM 4.0 to 5.0. In addition to providing more
complete C++17 support, this will allow us to experiment with
C++ coroutines.

Reviewed by: bapt
Differential Revision: https://reviews.freebsd.org/D12401