compiler: enable escape analysis for runtime

compiler: enable escape analysis for runtime

    The runtime package was hard-coded non-escape, and the escape
    analysis was not run for the runtime package. This CL removes
    the hard-code, and lets the escape analysis decide. It is not
    allowed for local variables and closures in the runtime to be
    heap allocated. This CL adds the check that make sure that they
    indeed do not escape.

    The escape analysis is always run when compiling the runtime
    now.

    Fixes golang/go#17431

    Reviewed-on: https://go-review.googlesource.com/86246

From-SVN: r256820