cmd/dist: use gohostarch for ssa rewrite check

Fix a build failure when bootstrapping the Go compiler with go-bootstrap 1.4
while the environment contains GOARCH=riscv64.

Building Go toolchain1 using go-1.4-bootstrap-20171003.
src/cmd/compile/internal/ssa/rewriteRISCV64.go:4814
invalid operation: y << x (shift count type int64, must be unsigned integer)

This is because:

 - buildtool.go:198: calls bootstrapRewriteFile(src)
 - bootstrapRewriteFile: buildtool.go:283 calls:
 - isUnneededSSARewriteFile: checks os.Getenv("GOARCH")
 - isUnneededSSARewriteFile: returns "", false
 - bootstrapRewriteFile: calls bootstrapFixImports
 - boostrapFixImports: generates code go1.4 cannot compile

Instead of checking "GOARCH" in the environment, use the gohostarch variable.

Change-Id: Ie9c190498555c4068461fead6278a62e924062c6
GitHub-Last-Rev: 300d7a7
GitHub-Pull-Request: #52362
Reviewed-on: https://go-review.googlesource.com/c/go/+/400376
Reviewed-by: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Auto-Submit: Bryan Mills <bcmills@google.com>
Run-TryBot: Bryan Mills <bcmills@google.com>
Reviewed-by: Joel Sing <joel@sing.id.au>
Run-TryBot: Joel Sing <joel@sing.id.au>

src/cmd/dist/buildtool.go

@@ -16,7 +16,6 @@ import (
 	"os"
 	"path/filepath"
 	"regexp"
-	"runtime"
 	"strings"
 )
 
@@ -243,11 +242,11 @@ var ssaRewriteFileSubstring = filepath.FromSlash("src/cmd/compile/internal/ssa/r
 
 // isUnneededSSARewriteFile reports whether srcFile is a
 // src/cmd/compile/internal/ssa/rewriteARCHNAME.go file for an
-// architecture that isn't for the current runtime.GOARCH.
+// architecture that isn't for the given GOARCH.
 //
 // When unneeded is true archCaps is the rewrite base filename without
 // the "rewrite" prefix or ".go" suffix: AMD64, 386, ARM, ARM64, etc.
-func isUnneededSSARewriteFile(srcFile string) (archCaps string, unneeded bool) {
+func isUnneededSSARewriteFile(srcFile, goArch string) (archCaps string, unneeded bool) {
 	if !strings.Contains(srcFile, ssaRewriteFileSubstring) {
 		return "", false
 	}
@@ -262,13 +261,10 @@ func isUnneededSSARewriteFile(srcFile string) (archCaps string, unneeded bool) {
 	archCaps = fileArch
 	fileArch = strings.ToLower(fileArch)
 	fileArch = strings.TrimSuffix(fileArch, "splitload")
-	if fileArch == os.Getenv("GOHOSTARCH") {
+	if fileArch == goArch {
 		return "", false
 	}
-	if fileArch == strings.TrimSuffix(runtime.GOARCH, "le") {
-		return "", false
-	}
-	if fileArch == strings.TrimSuffix(os.Getenv("GOARCH"), "le") {
+	if fileArch == strings.TrimSuffix(goArch, "le") {
 		return "", false
 	}
 	return archCaps, true
@@ -277,9 +273,9 @@ func isUnneededSSARewriteFile(srcFile string) (archCaps string, unneeded bool) {
 func bootstrapRewriteFile(srcFile string) string {
 	// During bootstrap, generate dummy rewrite files for
 	// irrelevant architectures. We only need to build a bootstrap
-	// binary that works for the current runtime.GOARCH.
+	// binary that works for the current gohostarch.
 	// This saves 6+ seconds of bootstrap.
-	if archCaps, ok := isUnneededSSARewriteFile(srcFile); ok {
+	if archCaps, ok := isUnneededSSARewriteFile(srcFile, gohostarch); ok {
 		return fmt.Sprintf(`// Code generated by go tool dist; DO NOT EDIT.
 
 package ssa