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

git-svn-id: svn+ssh://gcc.gnu.org/svn/gcc/trunk@248934 138bc75d-0d04-0410-961f-82ee72b054a4