compiler: let initializers hold addresses of fields

compiler: let initializers hold addresses of fields

    The runtime.dbgvars initializer looks like

    var dbgvars = []dbgVar{
            {"allocfreetrace", &debug.allocfreetrace},
    }

    Because the field address was not recognized as valid for a static
    initializer, the variable was initialized at runtime.  Normally that
    would be fine, but for the runtime package it meant that dbgvars was
    not initialized when it was read by parsedebugvars.  That meant that
    the GODEBUG environment variable did nothing.

    Fixing that revealed that the static initializer checks had to be more
    careful about interface types, just like the existing is_constant
    methods.

    Reviewed-on: https://go-review.googlesource.com/35891

From-SVN: r244982