compile, runtime: permit anonymous and empty fields in C header

    
    Permit putting structs with anonymous and empty fields in the C header
    file runtime.inc that is used to build the C runtime code.  This is
    required for upcoming 1.13 support, as the m struct has picked up an
    anonymous field.
    
    Doing this lets the C header contain all the type descriptor structs,
    so start using those in the C code.  This cuts the number of copies of
    type descriptor definitions from 3 to 2.
    
    Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/192343

From-SVN: r275227