cmd/go, plugin: disable plugins on darwin

We are seeing a bad stack map in #18190. In a copystack, it is
mistaking a slot for a pointer.

Presumably this is caused either by our fledgling dynlink support on
darwin, or a consequence of having two copies of the runtime in the
process. But I have been unable to work out which in the 1.8 window,
so pushing darwin support to 1.9 or later.

Change-Id: I7fa4d2dede75033d9a428f24c1837a4613bd2639
Reviewed-on: https://go-review.googlesource.com/34391Reviewed-by: Ian Lance Taylor <iant@golang.org>

doc/go1.8.html

@@ -417,7 +417,7 @@ version of gccgo.
   plugins written in Go, and a
   new <a href="/pkg/plugin/"><code>plugin</code></a> package for
   loading such plugins at run time. Plugin support is only currently
-  available on Linux and macOS.
+  available on Linux.
 </p>
 
 <h2 id="runtime">Runtime</h2>

doc/go1.8.txt

@@ -29,7 +29,6 @@ cmd/link: fix -buildmode=pie / -linkshared combination (CL 28996)
 cmd/link: for -buildmode=exe pass -no-pie to external linker (CL 33106)
 cmd/link: insert trampolines for too-far jumps on ARM (CL 29397)
 cmd/link: non-executable stack support for Solaris (CL 24142)
-cmd/link: plugin support on darwin/amd64 (CL 29394)
 cmd/link: put text at address 0x1000000 on darwin/amd64 (CL 32185)
 cmd/link: remove the -shared flag (CL 28852)
 cmd/link: split large elf text sections on ppc64x (CL 27790)

src/cmd/dist/test.go

@@ -786,8 +786,7 @@ func (t *tester) supportedBuildmode(mode string) bool {
 		// linux-arm64 is missing because it causes the external linker
 		// to crash, see https://golang.org/issue/17138
 		switch pair {
-		case "linux-386", "linux-amd64", "linux-arm",
-			"darwin-amd64":
+		case "linux-386", "linux-amd64", "linux-arm":
 			return true
 		}
 		return false

src/cmd/go/build.go

@@ -413,8 +413,7 @@ func buildModeInit() {
 		} else {
 			switch platform {
 			case "linux/amd64", "linux/arm", "linux/arm64", "linux/386",
-				"android/amd64", "android/arm", "android/arm64", "android/386",
-				"darwin/amd64":
+				"android/amd64", "android/arm", "android/arm64", "android/386":
 			default:
 				fatalf("-buildmode=plugin not supported on %s\n", platform)
 			}

src/plugin/plugin.go

@@ -4,7 +4,7 @@
 
 // Package plugin implements loading and symbol resolution of Go plugins.
 //
-// Currently plugins only work on Linux and Darwin.
+// Currently plugins only work on Linux.
 //
 // A plugin is a Go main package with exported functions and variables that
 // has been built with: