android: disable libunwind for now

There's an issue with dl_iterate_phdr().
JIT ARM: improve stack trace

Fix a problem at the boundary of DWARF-driven backtrace
and frame-driver backtrace, as well as handling of the
JIT's different internal calling conventions.

Also, allow crossing back from frame-driven to DWARF-driven.
Add MZ_TRY_EXTFLONUMS to OpenBSD.

Also add an error to unsupported platforms (copied from FreeBSD).

Merge to 5.3.4