watchOS: fixes for ilp32 ABI

[marionauta]

Xcode requires target amd64_32 (aarch64-watchos-ilp32) in order to build code for Apple Watches. This commit fixes compilation errors that appear when compiling with that target.

---

This PR is a followup to #21752

[alexrp]

This seems reasonable to me given the API shape of std.macho, but I'd like @kubkon to take a look at the std.debug.SelfInfo changes.

[kubkon]

I haven't looked closely yet, but do we really need all those intcasts sprinkled everywhere? Do we currently handle any 32bit arch for linux perhaps? If so how do we deal with the issue there?

[alexrp]

No, I've only just started the basics of x32/n32 support over in #21717. Standard library support is still on the to-do list. I believe that with these changes, aarch64-watchos-ilp32 will be the first ILP32-on-64 target that's actually usable with the standard library.

As I understand it, the int casts are necessary because of the API shape of std.c and std.macho:

• std.c.thread_state is defined in terms of u64s on aarch64, so when we want to put those register values into usize (u32) fields, we of course run into trouble. I checked the C headers and couldn't find any indication that this struct is different between aarch64-*-none and aarch64-*-ilp32. So I don't think we can do anything about this.
• For std.macho.section_64 the issue is similar. It's my understanding that aarch64-*-ilp32 on Darwin uses the 64-bit Mach-O format just like aarch64-*-none, so it doesn't seem like there's much to be done here either.

[marionauta]

As I understand it, the int casts are necessary because of the API shape of std.c and std.macho

Yeah, exactly that. I wasn't sure what was better, @intCast or @truncate, but I will apply the changes you suggested.

[marionauta]

with these changes, aarch64-watchos-ilp32 will be the first ILP32-on-64 target that's actually usable with the standard library.

Exciting! Just FYI, I've been using my branch to build static libraries (.a) for my watch game with not problems. And I've been using the standard library quite a lot.