The MBC3 and TAMA5 battery formats save the RTC data including a
`time_t` field which is the last field.

Since `time_t` is 32 bits for 32 bit builds and 64 bits for 64 bit
builds, pad it in the two battery structs with a `uint64_t` and detect
the 4 byte shorter saves made by older 32 bit builds.

Also remove some pointless code in save state reading that also uses
`sizeof(time_t)`.

Add two new constants for RTC data size in gbMemory.h and use them.

Signed-off-by: Rafael Kitover <rkitover@gmail.com>