cmd/gc: allow new conversion syntax

For consistency with conversions that look like function calls,
conversions that don't look like function calls now allow an
optional trailing comma.

That is, int(x,) has always been syntactically valid.
Now []int(x,) is valid too.

Fixes issue 4162.

R=ken2
CC=golang-dev
https://codereview.appspot.com/7288045