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