Support OpenBSD's native frame protection conventions.

OpenBSD's stack smashing protection differs slightly from other
platforms:

  1. The smash handler function is "__stack_smash_handler(const char
     *funcname)" instead of "__stack_chk_fail(void)".

  2. There's a hidden "long __guard_local" object that gets linked
     into each executable and DSO.

Patch by Matthew Dempsky.

llvm-svn: 183533