runtime: fix setting of isarchive

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