Add support for Nuxi CloudABI.

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

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@231681 91177308-0d34-0410-b5e6-96231b3b80d8