Rename DISABLE_TRACE to UWX_TRACE_ENABLE and change the conditional

Rename DISABLE_TRACE to UWX_TRACE_ENABLE and change the conditional
compilation accordingly. The net effect is that tracing is not by
default present and that it can easily be compiled-in.

The tracer uses getenv() and printf(), which cannot be used in all
cases (ie from the debugger) and with this change we improved the
applicability of the unwinder.

This change is made on the vendor branch and given back to the
vendor for possible inclusion in future versions.