cmd/internal/gc: change proginfo to return ProgInfo instead of writing to param

cmd/internal/gc: change proginfo to return ProgInfo instead of writing to param

This avoids the argument appearing to escape
(due to the fact that proginfo is always called
via a function pointer).

Change-Id: Ib9351ba18c80fd89e6a1d4f19dea386d4c657337
Reviewed-on: https://go-review.googlesource.com/6518
Reviewed-by: Rob Pike <r@golang.org>