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gba: handle 8bit writes to vram

ignore 8bit writes to obj and oam, and duplicate bits for bg and palette
memory, as per gbatek.

thanks aiju for helping with the implementation.

--- a/sys/src/games/gba/mem.c

+++ b/sys/src/games/gba/mem.c

@@ -315,6 +315,11 @@

 		return;

 	case 5:

 		b = a & sizeof(pram) - 1;

+		if(n == 1){

+			b &= ~1;

+			n = 2;

+			v |= v << 8;

+		}

 		cyc += (n+1) >> 1;

 		ar16write(pram + b/2, b & 1, v, n);

 		return;

@@ -322,13 +327,20 @@

 		b = a & 128*KB - 1;

 		if(b >= 64*KB)

 			b &= ~(32*KB);

+		if(n == 1 && ((reg[DISPCNT] & 7) > 2 && b < 80*KB || b < 64*KB)){

+			b &= ~1;

+			n = 2;

+			v |= v << 8;

+		}

 		cyc += (n+1) >> 1;

-		arwrite(vram + b, v, n);

+		if(n != 1)

+			arwrite(vram + b, v, n);

 		return;

 	case 7:

 		b = a & sizeof(oam) - 1;

 		cyc++;

-		ar16write(oam + b/2, b & 1, v, n);

+		if(n != 1)

+			ar16write(oam + b/2, b & 1, v, n);

 		return;

 	case 8: case 9: case 10: case 11: case 12: case 13:

 		if(backup == EEPROM){