stage2 ARM: amend implementation of various AIR instructions

- unwrap_errunion_err for registers
- unwrap_errunion_payload for registers
- ptr_slice_len_ptr for all MCValues
- ptr_slice_ptr_ptr for all MCValues

test/behavior/basic.zig

@@ -1060,7 +1060,6 @@ comptime {
 
 test "switch inside @as gets correct type" {
     if (builtin.zig_backend == .stage2_aarch64) return error.SkipZigTest; // TODO
-    if (builtin.zig_backend == .stage2_arm) return error.SkipZigTest; // TODO
 
     var a: u32 = 0;
     var b: [2]u32 = undefined;