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/galign.go b/src/cmd/8g/galign.go
index e92802d..1c03df5 100644
--- a/src/cmd/8g/galign.go
+++ b/src/cmd/8g/galign.go
@@ -54,6 +54,7 @@
 	case "387":
 		gc.Thearch.FREGMIN = x86.REG_F0
 		gc.Thearch.FREGMAX = x86.REG_F7
+		gc.Thearch.Use387 = true
 	case "sse2":
 		gc.Thearch.FREGMIN = x86.REG_X0
 		gc.Thearch.FREGMAX = x86.REG_X7
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)
diff --git a/src/cmd/8g/gsubr.go b/src/cmd/8g/gsubr.go
index 7ca4dac..929cbae 100644
--- a/src/cmd/8g/gsubr.go
+++ b/src/cmd/8g/gsubr.go
@@ -402,7 +402,7 @@
 	a := obj.AXXX
 	et := int(gc.Simtype[t.Etype])
 
-	if gc.Use_sse {
+	if !gc.Thearch.Use387 {
 		switch uint32(op)<<16 | uint32(et) {
 		default:
 			gc.Fatal("foptoas-sse: no entry %v-%v", gc.Oconv(int(op), 0), gc.Tconv(t, 0))
@@ -1036,10 +1036,10 @@
 
 	switch uint32(ft)<<16 | uint32(tt) {
 	default:
-		if gc.Use_sse {
-			floatmove_sse(f, t)
-		} else {
+		if gc.Thearch.Use387 {
 			floatmove_387(f, t)
+		} else {
+			floatmove_sse(f, t)
 		}
 		return