cmd/dist: redo flag-passing for bootstrap

This ought to revert the bad effects of
https://go-review.googlesource.com/#/c/20775/
If you don't pass BOOT_GO_GCFLAGS, you get the
old behavior.

Tweaked to allow multiple space-separated flags in
BOOT_GO_GCFLAGS.

Change-Id: I2a22a04211b4535d1c5a8ec7a8a78cb051161c31
Reviewed-on: https://go-review.googlesource.com/20871
Run-TryBot: David Chase <drchase@google.com>
Reviewed-by: Russ Cox <rsc@golang.org>

src/cmd/dist/build.go

@@ -167,7 +167,7 @@ func xinit() {
 		goextlinkenabled = b
 	}
 
-	gogcflags = os.Getenv("GO_GCFLAGS")
+	gogcflags = os.Getenv("BOOT_GO_GCFLAGS")
 
 	b = os.Getenv("CC")
 	if b == "" {
@@ -691,7 +691,7 @@ func install(dir string) {
 	}
 	compile := []string{pathf("%s/compile", tooldir), "-pack", "-o", b, "-p", pkg}
 	if gogcflags != "" {
-		compile = append(compile, gogcflags)
+		compile = append(compile, strings.Fields(gogcflags)...)
 	}
 	if dir == "runtime" {
 		compile = append(compile, "-+", "-asmhdr", pathf("%s/go_asm.h", workdir))

src/make.bash

@@ -151,8 +151,8 @@ if [ "$1" = "--no-clean" ]; then
 	buildall=""
 	shift
 fi
+./cmd/dist/dist bootstrap $buildall $GO_DISTFLAGS -v # builds go_bootstrap
 
-GO_GCFLAGS="$BOOT_GO_GCFLAGS" ./cmd/dist/dist bootstrap $buildall $GO_DISTFLAGS -v # builds go_bootstrap
 # Delay move of dist tool to now, because bootstrap may clear tool directory.
 mv cmd/dist/dist "$GOTOOLDIR"/dist
 echo