mem: Use a flag instead of address bit 63 for generic IPRs

mem: Use a flag instead of address bit 63 for generic IPRs

Using address bit 63 to identify generic IPRs caused problems on
SPARC, where IPRs are heavily used. This changeset redefines how
generic IPRs are identified. Instead of using bit 63, we now use a
separate flag (GENERIC_IPR) a memory request.