cmd/compile: eliminate direct uses of gc.Thearch in backends

This CL changes the GOARCH.Init functions to take gc.Thearch as a
parameter, which gc.Main supplies.

Additionally, the x86 backend is refactored to decide within Init
whether to use the 387 or SSE2 instruction generators, rather than for
each individual SSA Value/Block.

Passes toolstash-check -all.

Change-Id: Ie6305a6cd6f6ab4e89ecbb3cbbaf5ffd57057a24
Reviewed-on: https://go-review.googlesource.com/38301
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>

src/cmd/compile/main.go

@@ -20,33 +20,32 @@ import (
 	"os"
 )
 
+var archInits = map[string]func(*gc.Arch){
+	"386":      x86.Init,
+	"amd64":    amd64.Init,
+	"amd64p32": amd64.Init,
+	"arm":      arm.Init,
+	"arm64":    arm64.Init,
+	"mips":     mips.Init,
+	"mipsle":   mips.Init,
+	"mips64":   mips64.Init,
+	"mips64le": mips64.Init,
+	"ppc64":    ppc64.Init,
+	"ppc64le":  ppc64.Init,
+	"s390x":    s390x.Init,
+}
+
 func main() {
 	// disable timestamps for reproducible output
 	log.SetFlags(0)
 	log.SetPrefix("compile: ")
 
-	switch obj.GOARCH {
-	default:
+	archInit, ok := archInits[obj.GOARCH]
+	if !ok {
 		fmt.Fprintf(os.Stderr, "compile: unknown architecture %q\n", obj.GOARCH)
 		os.Exit(2)
-	case "386":
-		x86.Init()
-	case "amd64", "amd64p32":
-		amd64.Init()
-	case "arm":
-		arm.Init()
-	case "arm64":
-		arm64.Init()
-	case "mips", "mipsle":
-		mips.Init()
-	case "mips64", "mips64le":
-		mips64.Init()
-	case "ppc64", "ppc64le":
-		ppc64.Init()
-	case "s390x":
-		s390x.Init()
 	}
 
-	gc.Main()
+	gc.Main(archInit)
 	gc.Exit(0)
 }