When Go 1.3 is released, this will keep existing
Go 1.2 build scripts that use 'go tool pack grc' working.
For efficiency, such scripts should be changed to
use 6g -pack instead, but keeping the old behavior
available enables a more graceful transition.

LGTM=r
R=r
CC=golang-codereviews
https://golang.org/cl/66130043