cmd/dist, cmd/ld: GO_EXTLINK_ENABLED=0 defaults to -linkmode=internal

Change build system to set GO_EXTLINK_ENABLED=0 by default for
OS X 10.6, since the system linker has a bug and can not
handle the object files generated by 6l.

Fixes issue 5130.

R=golang-dev, r
CC=golang-dev
https://codereview.appspot.com/8183043