[libunwind] Increase the external value of _LIBUNWIND_CURSOR_SIZE for SEH/x86_64

For x86_64 Windows targets (that use SEH), _LIBUNWIND_CURSOR_SIZE
is 204; this fixes corruption in test cases that include libunwind.h
without manually defining _LIBUNWIND_IS_NATIVE_ONLY.

If the libunwind.h header is included without defining
_LIBUNWIND_IS_NATIVE_ONLY (like in the libunwind test cases), the
sizes are set to accommodate the maximum possible cursors and
contexts.

(Alternatively, __libunwind_config.h should be changed to default
to native unwinding unless cross unwinding has been requested.
Cross unwinding isn't implemented as far as I know anyway.)

Differential Revision: https://reviews.llvm.org/D147634