Add support for Nuxi CloudABI.

CloudABI is a POSIX-like runtime environment built around the concept of
capability-based security. More details:

	https://github.com/NuxiNL/cloudlibc

CloudABI uses its own ELFOSABI number. This number has been allocated by
the maintainers of ELF a couple of days ago.

Reviewed by:	echristo

llvm-svn: 231681