Identify various "????"s

[Rangi42]

Corresponds to pret/pokegold#116

The remaining "???"s are mostly new to Crystal (mobile-related):

audio/engine.asm:2208:	; add ??? to the next result
audio/engine.asm:2217:	; store result in ???
audio/engine.asm:2286:	; clear ???
data/battle_tower/unknown.asm:1:BattleTowerTrainerData:: ; ???
data/player_names.asm:6:	db 0 ; ???
data/player_names.asm:25:	db 0 ; ???
engine/events/battle_tower/battle_tower.asm:1387:	ld a, 0 ; ???
engine/menus/init_gender.asm:14:	res 0, a ; ???
engine/menus/init_gender.asm:17:	res 1, a ; ???
lib/mobile/main.asm:8528:	cp $8e ; LOW(MD5_K_Table + $100) ???
mobile/mobile_40.asm:242:	ret ; ???
mobile/mobile_41.asm:1034:	ld b, $0f ; no graphics at 0f:40b0; JP leftover???
mobile/mobile_46.asm:1972:	ld a, $3 ; ???
mobile/mobile_46.asm:2501:	ld a, $6 ; ???