cmd/dist: disable PGO for toolchain2 build

Toolchain2 is only used for building toolchain3. We don't need to
build it with PGO. And building with PGO causes packages to be
built twice (one with PGO for the compiler, one without for other
programs). Disable PGO for toolchain2.

Also, I thought cmd/dist requires toolchain2 and toolchain3
compilers are identical binaries, so they need to be built in the
same way. But it doesn't.

Change-Id: Iaf49816da3dd06db79b48482c0e2435e09b512d7
Reviewed-on: https://go-review.googlesource.com/c/go/+/501335
TryBot-Result: Gopher Robot <[email protected]>
Reviewed-by: Michael Pratt <[email protected]>
Reviewed-by: Bryan Mills <[email protected]>
Run-TryBot: Cherry Mui <[email protected]>

Author: cherrymui

src/cmd/dist/build.go

@@ -1457,7 +1457,8 @@ func cmdbootstrap() {
 	os.Setenv("CC", compilerEnvLookup("CC", defaultcc, goos, goarch))
 	// Now that cmd/go is in charge of the build process, enable GOEXPERIMENT.
 	os.Setenv("GOEXPERIMENT", goexperiment)
-	goInstall(toolenv(), goBootstrap, toolchain...)
+	// No need to enable PGO for toolchain2.
+	goInstall(toolenv(), goBootstrap, append([]string{"-pgo=off"}, toolchain...)...)
 	if debug {
 		run("", ShowOutput|CheckExit, pathf("%s/compile", tooldir), "-V=full")
 		copyfile(pathf("%s/compile2", tooldir), pathf("%s/compile", tooldir), writeExec)