[Triple] Define OS Check for Haiku

This adds the OS check for the Haiku operating system, as it was
missing in the Triple class. Tests for x86_64-unknown-haiku and
i586-pc-haiku were also added.

These patches only affect Haiku and are completely harmless for
other platforms.

Patch by Calvin Hill <calvin@hakobaito.co.uk>

llvm-svn: 311153