[ExecutionEngine] Check for libunwind before calling __register_frame

libgcc and libunwind have different flavours of __register_frame. Both
 flavours are already correctly handled, except that the code to handle
the libunwind flavour is guarded by __APPLE__. This change uses the
presence of __unw_add_dynamic_fde in libunwind instead to detect whether
libunwind is used, rather than hardcoding it as Apple vs. non-Apple.

Fixes PR44074.

Thanks to Albert Jin <albert.jin@gmail.com> and Chris Schafmeister
<chris.schaf@verizon.net> for identifying the problem.

Reviewed By: lhames

Differential Revision: https://reviews.llvm.org/D106129