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 issue 4462.

R=rsc, iant, r, ken, lvd
CC=golang-dev
https://codereview.appspot.com/6861043

h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang