cmd/8g, cmd/internal/gc: fix GO386=387 build

Adjust Thearch.FREG_MIN/MAX when using non sse2 mode in 8g.

Also, gc.Use_sse is treated as a bool, so make it a bool.

Change-Id: I840411605344bb31c32f492b3e6729166c084f0c
Reviewed-on: https://go-review.googlesource.com/7993
Reviewed-by: Aram Hăvărneanu <aram@mgk.ro>
Reviewed-by: Rob Pike <r@golang.org>
Reviewed-by: Dave Cheney <dave@cheney.net>
Run-TryBot: Dave Cheney <dave@cheney.net>
diff --git a/src/cmd/internal/gc/gsubr.go b/src/cmd/internal/gc/gsubr.go
index 05642d6..4a9f895 100644
--- a/src/cmd/internal/gc/gsubr.go
+++ b/src/cmd/internal/gc/gsubr.go
@@ -692,6 +692,10 @@
 		Fatal("out of fixed registers")
 
 	case TFLOAT32, TFLOAT64:
+		if Thearch.Thechar == '8' && !Use_sse {
+			i = Thearch.FREGMIN // x86.REG_F0
+			break Switch
+		}
 		if o != nil && o.Op == OREGISTER {
 			i = int(o.Val.U.Reg)
 			if Thearch.FREGMIN <= i && i <= Thearch.FREGMAX {