[libunwind] Ensure enough alignment for unw_cursor_t for SEH build configurations

[libunwind] Ensure enough alignment for unw_cursor_t for SEH build configurations

When built in SEH mode, UnwindCursor contains a CONTEXT struct,
which is aligned to 16 bytes in most configurations, causing the
whole UnwindCursor object to have 16 byte alignment.

This fixes backtraces using _Unwind_Backtrace on x86_64 mingw,
where an unw_cursor_t allocated on the stack was misaligned before.

This is an ABI break for this struct for this configuration, but very
few callers call libunwind directly (and even fewer directly allocate
an unw_cursor_t anyway).

Differential Revision: https://reviews.llvm.org/D86102