Add target information for CloudABI on x86-64.

CloudABI can be identified by the __CloudABI__ preprocessor definition. The
system uses ELF executables.

CloudABI uses Unicode 7.0.0 for the encoding of wchar_t. As Unicode 7.0.0 is
synchronized with ISO/IEC 10646:2012 (released on 2012-06-01),
__STDC_ISO_10646__ is defined as 201206L.

llvm-svn: 231912