runtime: install SIGURG handler on C created threads

    
    Precise stack scan uses SIGURG to trigger a stack scan. We need
    to have Go signal handler installed for SIGURG.
    
    Reviewed-on: https://go-review.googlesource.com/c/159097

From-SVN: r268230

gcc/go/gofrontend/MERGE

-e3271f3e09337b951822ba5c596d8cfe3b94508e
+d67c4bf0c42b79d54925ba8c5f23278ee6c3efb6
 
 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

@@ -142,8 +142,8 @@ func sigInstallGoHandler(sig uint32) bool {
 	}
 
 	// When built using c-archive or c-shared, only install signal
-	// handlers for synchronous signals and SIGPIPE.
-	if (isarchive || islibrary) && t.flags&_SigPanic == 0 && sig != _SIGPIPE {
+	// handlers for synchronous signals, SIGPIPE, and SIGURG.
+	if (isarchive || islibrary) && t.flags&_SigPanic == 0 && sig != _SIGPIPE && sig != _SIGURG {
 		return false
 	}