spec: clarify expression statements

spec: clarify expression statements

Function and method calls are valid expression statements,
but calling certain built-in functions is not permitted.
Enumerate the built-ins.

Also: unsafe.Offsetof permits parenthesized selectors as
arguments.

This is simply documenting existing compiler behavior
(both gc and gccgo agree).

R=r, rsc, iant, ken
CC=golang-dev
https://golang.org/cl/6498138