cmd/dist: default to clang, not gcc, on freebsd

Fixes #11380.

Change-Id: I0a284ad2a46826ce82486479ea4e79f0f470292f
Reviewed-on: https://go-review.googlesource.com/16635Reviewed-by: Ian Lance Taylor <iant@golang.org>

doc/go1.6.txt

 Tools:
 
+cmd/dist: use clang on FreeBSD (https://golang.org/cl/16635)
 cmd/go: vendoring enabled by default (https://golang.org/cl/13967/)
 cmd/go: flags for tests must precede package name if present; also makes it easier to pass flags to test binaries (https://golang.org/cl/14826)
 cmd/go: add -msan option (https://golang.org/cl/16169)

src/cmd/dist/util.go

@@ -404,6 +404,9 @@ func main() {
 		if strings.Contains(run("", CheckExit, "sysctl", "machdep.cpu.extfeatures"), "EM64T") {
 			gohostarch = "amd64"
 		}
+	case "freebsd":
+		// Since FreeBSD 10 gcc is no longer part of the base system.
+		defaultclang = true
 	case "solaris":
 		// Even on 64-bit platform, solaris uname -m prints i86pc.
 		out := run("", CheckExit, "isainfo", "-n")