plugin: add freebsd/amd64 plugin support

Change-Id: I4e7b0e99fd0f6f7d6a8ef60e9d3cb5baeb80a2b6
Reviewed-on: https://go-review.googlesource.com/c/go/+/191617
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/cmd/dist/test.go

@@ -972,6 +972,8 @@ func (t *tester) supportedBuildmode(mode string) bool {
 			return true
 		case "darwin-amd64":
 			return true
+		case "freebsd-amd64":
+			return true
 		}
 		return false
 	case "pie":

src/cmd/go/internal/work/init.go

@@ -197,6 +197,7 @@ func buildModeInit() {
 			case "darwin/amd64":
 				// Skip DWARF generation due to #21647
 				forcedLdflags = append(forcedLdflags, "-w")
+			case "freebsd/amd64":
 			default:
 				base.Fatalf("-buildmode=plugin not supported on %s\n", platform)
 			}

src/cmd/link/internal/ld/config.go

@@ -95,7 +95,7 @@ func (mode *BuildMode) Set(s string) error {
 			default:
 				return badmode()
 			}
-		case "darwin":
+		case "darwin", "freebsd":
 			switch objabi.GOARCH {
 			case "amd64":
 			default:

src/plugin/plugin.go

@@ -13,7 +13,7 @@
 // already part of the program are called. The main function is not run.
 // A plugin is only initialized once, and cannot be closed.
 //
-// Currently plugins are only supported on Linux and macOS.
+// Currently plugins are only supported on Linux, FreeBSD, and macOS.
 // Please report any issues.
 package plugin
 

src/plugin/plugin_dlopen.go

@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// +build linux,cgo darwin,cgo
+// +build linux,cgo darwin,cgo freebsd,cgo
 
 package plugin
 

src/plugin/plugin_stubs.go

@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// +build !linux,!darwin !cgo
+// +build !linux,!freebsd,!darwin !cgo
 
 package plugin