cmd/gc: make forward declaration in pure Go package an error

cmd/gc: make forward declaration in pure Go package an error

An error during the compilation can be more precise
than an error at link time.

For 'func init', the error happens always: you can't forward
declare an init func because the name gets mangled.

For other funcs, the error happens only with the special
(and never used by hand) -= flag, which tells 6g the
package is pure go.

The go command now passes -= for pure Go packages.

Fixes #3705.

R=ken2
CC=golang-dev
https://golang.org/cl/6996054