feat(cpu): implement format 12 thumb instructions

src/cpu/thumb/format12.zig

@@ -0,0 +1,16 @@
+const std = @import("std");
+
+const Bus = @import("../../Bus.zig");
+const Arm7tdmi = @import("../../cpu.zig").Arm7tdmi;
+const InstrFn = @import("../../cpu.zig").ThumbInstrFn;
+
+pub fn format12(comptime isSP: bool, comptime rd: u3) InstrFn {
+    return struct {
+        fn inner(cpu: *Arm7tdmi, _: *Bus, opcode: u16) void {
+            const left = if (isSP) cpu.r[13] else cpu.r[15] + 2 & 0xFFFF_FFFD; // fetch (+2)
+            const right = @truncate(u10, opcode & 0xFF) << 2;
+            const result = left + right; // TODO: What about overflows?
+            cpu.r[rd] = result;
+        }
+    }.inner;
+}