spec: clarify use of built-ins in go/defer statements

spec: clarify use of built-ins in go/defer statements

1) Built-ins are restricted like for expression statements.
   This documents the status quo.

2) Calls cannot be parenthesized. The spec is not clear. gccgo
   permits it already, gc doesn't. Be explicit in the spec.

 Fixes #4462.

R=rsc, iant, r, ken, lvd
CC=golang-dev
https://golang.org/cl/6861043