cmd/go: assume that code in $GOROOT is up to date
cmd/go: assume that code in $GOROOT is up to date

Do not check compiler/linker timestamps for packages that are in the
$GOROOT. Avoids trying to rebuild non-writable standard packages when
timestamps have not been retained on the Go binaries.

Fixes issue 4106.

R=golang-dev, rsc
CC=golang-dev
http://codereview.appspot.com/6533053

h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang