Add support for SEH unwinding on Windows.

Summary:
I've tested this implementation on x86-64 to ensure that it works. All
`libc++abi` tests pass, as do all `libc++` exception-related tests. ARM
still remains to be implemented (@compnerd?).

Special thanks to KJK::Hyperion for his excellent series of articles on
how EH works on x86-64 Windows. (Seriously, check it out. It's awesome.)

I'm actually not sure if this should go in as is. I particularly don't
like that I duplicated the UnwindCursor class for this special case.

Reviewers: mstorsjo, rnk, compnerd, smeenai, javed.absar

Subscribers: mgorny, kristof.beyls, christof, chrib, cfe-commits, compnerd, llvm-commits

Differential Revision: https://reviews.llvm.org/D50564

git-svn-id: https://llvm.org/svn/llvm-project/libunwind/trunk@341125 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/src/UnwindLevel1-gcc-ext.c b/src/UnwindLevel1-gcc-ext.c
index 38f141e..1c66265 100644
--- a/src/UnwindLevel1-gcc-ext.c
+++ b/src/UnwindLevel1-gcc-ext.c
@@ -25,6 +25,10 @@
 
 #if defined(_LIBUNWIND_BUILD_ZERO_COST_APIS)
 
+#if defined(_LIBUNWIND_SUPPORT_SEH_UNWIND)
+#define private_1 private_[0]
+#endif
+
 ///  Called by __cxa_rethrow().
 _LIBUNWIND_EXPORT _Unwind_Reason_Code
 _Unwind_Resume_or_Rethrow(_Unwind_Exception *exception_object) {