cmd/8g, cmd/internal/gc: clean up GO386 handling

This change cleans up some of the uglyness introduced in 8fc73a39efe1
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/7994
Reviewed-by: Russ Cox <rsc@golang.org>
diff --git a/src/cmd/8g/ggen.go b/src/cmd/8g/ggen.go
index 949156e..a4a373c 100644
--- a/src/cmd/8g/ggen.go
+++ b/src/cmd/8g/ggen.go
@@ -617,10 +617,10 @@
 		return
 	}
 
-	if gc.Use_sse {
-		cgen_floatsse(n, res)
-	} else {
+	if gc.Thearch.Use387 {
 		cgen_float387(n, res)
+	} else {
+		cgen_floatsse(n, res)
 	}
 }
 
@@ -761,7 +761,7 @@
 	var et int
 	var n2 gc.Node
 	var ax gc.Node
-	if gc.Use_sse {
+	if !gc.Thearch.Use387 {
 		if nl.Addable == 0 {
 			var n1 gc.Node
 			gc.Tempname(&n1, nl.Type)