chore: introduce zba-util

In an effort to reuse code between zba and zba-gdbstub, move common util
code (like the SPSC Channel I implemented in this commit) in a new lib

src/core/cpu/thumb/data_transfer.zig

@@ -2,8 +2,8 @@ const Bus = @import("../../Bus.zig");
 const Arm7tdmi = @import("../../cpu.zig").Arm7tdmi;
 const InstrFn = @import("../../cpu.zig").thumb.InstrFn;
 
-const rotr = @import("../../../util.zig").rotr;
-const sext = @import("../../../util.zig").sext;
+const rotr = @import("zba-util").rotr;
+const sext = @import("zba-util").sext;
 
 pub fn fmt6(comptime rd: u3) InstrFn {
     return struct {