Some operating systems (e.g. Linux, FreeBSD) provide the
_Unwind_Backtrace() function in libgcc_s.so, which is automatically
linked into any binary using the functions provided by that library.  On
OpenBSD, though, _Unwind_Backtrace() is provided by libc++abi.so, which
is not automatically linked into binaries produced by the stock system C
compiler.

Meanwhile, lib/isc/backtrace.c assumes that any GNU-compatible toolchain
allows _Unwind_Backtrace() to be used without any extra provisions in
the build system.  This causes build failures on OpenBSD (and possibly
other systems).

Instead of making assumptions, actually check for _Unwind_Backtrace()
support in the toolchain if the backtrace() function is unavailable.

(cherry picked from commit 10d70557)