[OpenBSD] Add type sign information for OpenBSD

Like NetBSD, OpenBSD prefers having a consistent set of typedefs
across the architectures it supports over strictly following the ARM
ABIs.  The diff below makes sure that clang's view of those types
matches OpenBSD's system header files.  It also adds a test that
checks the relevant types on all OpenBSD platforms that clang works
on.  Hopefully we can add mips64 and powerpc to that list in the
future.

Patch by Mark Kettenis <mark.kettenis@xs4all.nl>

llvm-svn: 282184