cmd/go: permit installing into a subdirectory of $GOPATH/bin

In https://golang.org/cl/12080 we forbade installing cross-compiled
binaries into a subdirectory of $GOBIN, in order to fix
https://golang.org/issue/9769.  However, that fix was too aggressive,
in that it also forbade installing into a subdirectory of $GOPATH/bin.

This patch permits installing cross-compiled binaries into a
subdirectory $GOPATH/bin while continuing to forbid installing into a
subdirectory of $GOBIN.

Fixes #11778.

Change-Id: Ibc9919554e8c275beff54ec8bf919cfaa03b11ba
Reviewed-on: https://go-review.googlesource.com/12938
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>

src/cmd/go/go_test.go

@@ -765,7 +765,7 @@ func TestGoInstallErrorOnCrossCompileToBin(t *testing.T) {
 	}
 	tg.setenv("GOOS", "linux")
 	tg.setenv("GOARCH", goarch)
-	tg.runFail("install", "mycmd")
+	tg.run("install", "mycmd")
 	tg.setenv("GOBIN", tg.path("."))
 	tg.runFail("install", "mycmd")
 	tg.run("install", "cmd/pack")

src/cmd/go/pkg.go

@@ -744,8 +744,8 @@ func (p *Package) load(stk *importStack, bp *build.Package, err error) *Package
 		} else if p.build.BinDir != "" {
 			// Install to GOBIN or bin of GOPATH entry.
 			p.target = filepath.Join(p.build.BinDir, elem)
-			if !p.Goroot && strings.Contains(elem, "/") {
-				// Do not create bin/goos_goarch/elem.
+			if !p.Goroot && strings.Contains(elem, "/") && gobin != "" {
+				// Do not create $GOBIN/goos_goarch/elem.
 				p.target = ""
 				p.gobinSubdir = true
 			}