Change way OAM memory is read on ARM to take advantage of quick bitfield extraction.
ARM can extract and sign-extend bit fields with one instruction. The old method would do this all
at the front of the function into a memory structure, the new method uses C bitfield structures (like
many of the GPU structures already do).

Before:
I/libretro(11307): [PERF]: Performance counters:
I/libretro(11307): [PERF]: Avg (_spriteRenderP): 13522 ticks, 190530 runs.

After:
I/libretro(10624): [PERF]: Performance counters:
I/libretro(10624): [PERF]: Avg (_spriteRenderP): 8671 ticks, 190146 runs.