runtime: fix setting of isarchive

When I updated to Go 1.8, the initsig function moved from
    signal1_unix.go to signal_unix.go, and lost a gccgo-specific change in
    the move.  Add it back.
    
    This would have been caught by the misc/cgo/testcarchive tests in the
    gc repository, but we don't run those in the gccgo repository.  We
    should fix that, somehow.
    
    Reviewed-on: https://go-review.googlesource.com/35839

From-SVN: r244947

gcc/go/gofrontend/MERGE

@@ -1,4 +1,4 @@
-f8c451ed42fd71b85afab54a33fc321df0ff3b0b
+2c62d5223e814887801b1540162c72b90299d910
 
 The first line of this file holds the git revision number of the last
 merge done from the gofrontend repository.

libgo/go/runtime/signal_unix.go

@@ -66,6 +66,11 @@ 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