This change cleans up some of the uglyness introduced in 8fc73a39
by moving the gc.Use_sse into the gc.Arch struct and adjusting its
zero value to be more useful.

Change-Id: I26ff5d9ac57b3f25e936519e443de6583cdafa56
Reviewed-on: https://go-review.googlesource.com/7994Reviewed-by: Russ Cox <rsc@golang.org>