all.rc: make sure the Go tools end up in /bin

At least one test (in package runtime) depends
on `go' being in $path. We simply bind GOROOT/bin
before /bin to make sure the latest copy of the
binary is accessible there.

R=rsc, rminnich, ality
CC=golang-dev
https://golang.org/cl/7391047
diff --git a/src/all.rc b/src/all.rc
index 04d4b25..b9d4c32 100755
--- a/src/all.rc
+++ b/src/all.rc
@@ -9,5 +9,6 @@
 }
 
 . ./make.rc --no-banner
+bind -b $GOROOT/bin /bin
 ./run.rc --no-rebuild
 $GOTOOLDIR/dist banner  # print build info