runtime: set isarchive in initsig

The library initialization code in go-libmain.c sets the C variable
    runtime_isarchive but failed to set the Go variable runtime.isarchive.
    We don't currently have a way to let C code access an unexported Go
    variable, but fortunately the only time the Go function initsig is
    called with an argument of true is exactly where we want to set
    isarchive.  So let initsig do it.
    
    Reviewed-on: https://go-review.googlesource.com/33753

From-SVN: r243094

gcc/go/gofrontend/MERGE

@@ -1,4 +1,4 @@
-9be198d960e4bc46e21e4da1e3d4a1619266b8ab
+97b949f249515a61d3c09e9e06f08c8af189e967
 
 The first line of this file holds the git revision number of the last
 merge done from the gofrontend repository.

libgo/go/runtime/signal1_unix.go

@@ -65,6 +65,11 @@ var signalsOK bool
 //go:nosplit
 //go:nowritebarrierrec
 func initsig(preinit bool) {
+	if preinit {
+		// preinit is only passed as true if isarchive should be true.
+		isarchive = true
+	}
+
 	if !preinit {
 		// It's now OK for signal handlers to run.
 		signalsOK = true