runtime: simplify GOOS detection in mstart()

The existing condition is long and repetitive.  Using select/case with
multiple values in the expression list is more concise and clearer.

Change-Id: I43f8abcf958e433468728f1d89ff1436332b29da
Reviewed-on: https://go-review.googlesource.com/c/go/+/188519Reviewed-by: Keith Randall <khr@golang.org>
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/runtime/proc.go

@@ -1167,7 +1167,8 @@ func mstart() {
 	mstart1()
 
 	// Exit this thread.
-	if GOOS == "windows" || GOOS == "solaris" || GOOS == "illumos" || GOOS == "plan9" || GOOS == "darwin" || GOOS == "aix" {
+	switch GOOS {
+	case "windows", "solaris", "illumos", "plan9", "darwin", "aix":
 		// Windows, Solaris, illumos, Darwin, AIX and Plan 9 always system-allocate
 		// the stack, but put it in _g_.stack before mstart,
 		// so the logic above hasn't set osStack yet.