Upgrade the CloudABI toolchain and utilities.

A new version of cloudlibc and cloudabi-utils has been released, to
catch up with ABI changes (due to the import into FreeBSD HEAD). Bump
these ports up to v0.4.

While there, upgrade the LLVM packages. It looks like the LLVM project
is finally releasing tarballs itself, meaning we can use those instead
of requiring me to host them myself. Update the ports to use a common
Makefile (cloudabi-libc++/Makefile.common) to set the PORTVERSION,
MASTER_SITES, etc.

Reviewed by: bapt
Differential Revision: https://reviews.freebsd.org/D3332