compiler: typing fixes for Interface_mtable_expression

compiler: typing fixes for Interface_mtable_expression

    Interface_mtable_expression::do_type computes a type that incorporates
    Go type descriptors for the interface methods, whereas in order to
    have strict type agreement with the mtable data, the interface method
    fields need to be C function ptrs. Change the type recipe accordingly,
    and then update Interface_mtable_expression::do_get_backend to compute
    a revised backend type that uses the correct fcn types.

    Reviewed-on: https://go-review.googlesource.com/44750

From-SVN: r248934