[libunwind][include] Add SEH declarations to <unwind.h>.

Summary:
Make the `_Unwind_Exception` struct correct under SEH. Add a
declaration of `_GCC_specific_handler()`, which is used by SEH versions
of Itanium personality handlers to do common setup. Roughly corresponds
to Clang's D50380.

Reviewers: mstorsjo, rnk, compnerd, smeenai

Subscribers: christof, chrib, cfe-commits, llvm-commits

Differential Revision: https://reviews.llvm.org/D50414

llvm-svn: 339258