cmd/pack: stop parsing object files to generate __.SYMDEF
cmd/pack: stop parsing object files to generate __.SYMDEF

Nothing looks at __.SYMDEF, and the object file format is changing.

R=golang-dev, r
CC=golang-dev
https://codereview.appspot.com/39580044

pombredanne/go-deleteme,pombredanne/go-deleteme,pombredanne/go-deleteme,pombredanne/go-deleteme,tv42/old-go,pombredanne/go-deleteme,tv42/old-go,tv42/old-go,tv42/old-go,pombredanne/go-deleteme,tv42/old-go,tv42/old-go,tv42/old-go,pombredanne/go-deleteme