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

include/llvm/MC/MCELFObjectWriter.h

@@ -41,6 +41,8 @@ protected:
 public:
   static uint8_t getOSABI(Triple::OSType OSType) {
     switch (OSType) {
+      case Triple::CloudABI:
+        return ELF::ELFOSABI_CLOUDABI;
       case Triple::PS4:
       case Triple::FreeBSD:
         return ELF::ELFOSABI_FREEBSD;