Moves runtime/trace support (including its command line flag) behind
a "gc" build tag to allow goimports to build under gccgo, which does
not support runtime/trace.
Updates golang/go#15544.
Change-Id: I017a44089c0a4f3d3ba98815d57a141e25b3fe56
Reviewed-on: https://go-review.googlesource.com/26998
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
