runtime: fix sigfwd to not allocate memory

runtime: fix sigfwd to not allocate memory

    The use of &[1]uintptr{fn} was causing sigfwd to allocate memory, even
    though it is being compiled for the runtime package.  That is a bad
    idea for this function, which is invoked by a signal handler.  Rewrite
    it to use only constructs that do not allocate memory when compiled
    for the runtime package.

    The test for this is misc/cgo/testcarchive in the main repo, which we
    don't yet test.

    Reviewed-on: https://go-review.googlesource.com/37454

From-SVN: r245777