Move memory_map_[read|write] and reg to memory.c

This is not MIPS-specific, is defined in memory.h and used
everywhere in memory.c, so it'd better be in memory.c

Plus, it seems to fix a segmentation fault (related to alignment?).
Revert "Move memory_map_[read|write] and reg to memory.c"

mips/stub.S expect those three buffers to be contiguous in
memory. Moving them to C does not ensure this.

This reverts commit d91116e4bcf39054986bb4b8242744caf75a14f7.
MIPS: Move memory_map_[read|write] and reg to the BSS section

They were previously on the TEXT section, which is read-only
on systems that feature memory protection like on the GCW Zero.
Merge https://github.com/pcercuei/ReGBA