Add missing _US_ACTION_MASK constant to unwind.h

Summary:
During building of recent compiler-rt sources on FreeBSD for arm, I
noticed that our unwind.h (which originates in libunwind) was missing
the `_US_ACTION_MASK` constant:

    compiler-rt/lib/builtins/gcc_personality_v0.c:187:18: error: use of undeclared identifier '_US_ACTION_MASK'
        if ((state & _US_ACTION_MASK) != _US_UNWIND_FRAME_STARTING)
                     ^

It appears that both clang's internal unwind.h, and libgcc's unwind.h
define this constant as 3, so let's add this to libunwind's version too.

Reviewers: logan, kledzik, davide, emaste

Subscribers: joerg, davide, aemerson, emaste, llvm-commits

Differential Revision: https://reviews.llvm.org/D24222

llvm-svn: 280669