runtime: replace trace.enabled with traceEnabled

runtime: replace trace.enabled with traceEnabled

[git-generate]
cd src/runtime
grep -l 'trace\.enabled' *.go | grep -v "trace.go" | xargs sed -i 's/trace\.enabled/traceEnabled()/g'

Change-Id: I14c7821c1134690b18c8abc0edd27abcdabcad72
Reviewed-on: https://go-review.googlesource.com/c/go/+/494181
Run-TryBot: Michael Knyszek <mknyszek@google.com>
Auto-Submit: Michael Knyszek <mknyszek@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Michael Pratt <mpratt@google.com>