Also update cmd/dist to avoid setting gcflags and ldflags explicitly
when the set of flags to be set is empty (a verbose way of specifying
the default behavior).
Stamping was disabled for the Go standard library in CL 356014 due to
the cmd/dist flags causing cmd/go to (correctly) report the resulting
binaries as stale.
With cmd/dist fixed, we can also remove the special case in cmd/go,
which will allow tests of binaries in 'cmd' to read the build info
embedded in the test binary. That build info may be useful to
determine (say) whether runtime.GOROOT ought to work without GOROOT
set in the environment.