cmd/compile: improve error message if init is directly invoked

cmd/compile: improve error message if init is directly invoked

Fixes #8481.

Inform the user that init functions cannot be directly invoked
in user code, as mandated by the spec at:
http://golang.org/ref/spec#Program_initialization_and_execution.

Change-Id: Ib12c0c08718ffd48b76b6f9b13c76bb6612d2e7b
Reviewed-on: https://go-review.googlesource.com/34790
Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>