Avoid constant address resolution via a pointer in memory.c without -flto

GCC cannot identify the rdramb pointer as const and therefore not remove any
indirection when using it without link-time optimization. The missing const
reduces the performance slightly in (read|write)_rdram[bhd]? because extra
instructions have to be generated to get the value of rdramb from memory.

Reported-by: Nebuleon Fumika