Log Message:
emulators/libretro-mgba: Update to 0.7.1

Bugfixes:
 - GB: Fix crash when accessing SRAM if no save loaded and cartridge has no SRAM
 - GB Serialize: Fix loading states with negative pixel x (fixes mgba.io/i/1293)
 - GB, GBA Serialize: Fix loading two states in a row
 - GBA: Fix video timing when skipping BIOS (fixes mgba.io/i/1318)
 - GBA DMA: Fix Display Start DMAs
 - GBA DMA: Fix DMA start/end timing
 - GBA DMA: Fix invalid DMA handling (fixes mgba.io/i/1301)
 - GBA Memory: Fix a few AGBPrint crashes
 - GBA Memory: Fix OOB ROM reads showing up as AGBPrint memory
 - GBA SIO: Prevent writing read-only multiplayer bits
 - GBA Video: Fix enabling layers in non-tile modes (fixes mgba.io/i/1317)
Misc:
 - GBA Video: Improve sprite cycle counting (fixes mgba.io/i/1274)