backtrace: Opt-out of backtrace support with -DTRACE_BACKTRACE=0

For apitrace integrated into Regal we want to opt-out of backtrace support, for now.
One reason is that it's fairly platform-specific.  Another concern is potential
build issue across various Linux flavors.  This change ought to make no difference
with TRACE_BACKTRACE undefined at build-time.