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