Fix build of si with base gcc on i386

Casting from rman_res_t to a pointer results in "cast to pointer from
integer of different size" warnings with base gcc on i386, so use
intermediate casts to uintptr_t to suppress these.

Direct commit to stable/11, since si(4) has been removed from head.

Reviewed by: jhb
Differential Revision: https://reviews.freebsd.org/D15752