Support dynamically-linked NetBSD libc when cross-compiling

[alexrp]

libc update instructions: https://github.com/ziglang/zig/wiki/Updating-libc#netbsd

ABI tooling: https://github.com/ziglang/libc-abi-tools/tree/main/netbsd

Caveats:

• This only works for 10.1+ (we still target 9.4 by default).
• As with glibc and musl, we only ship the latest system and libc headers (10.1 specifically).
• We don't support .init/.fini because it's 2025 and people should be using .init_array/.fini_array.
• I removed some silly GCJ support code from crt1.

Contributes to #2877.

Release Notes

Zig now allows cross-compiling to NetBSD 10.1+ by providing stub libraries for dynamic libc, similar to how cross-compilation for glibc is handled. Additionally, all system and libc headers are provided.

[alexrp]

@andrewrk FYI, the new NetBSD headers and sources add up to ~12M (uncompressed), so a little less than what we saw for FreeBSD in #23835.

[alexrp]

I should note that libc++ currently fails to cross-compile for NetBSD even with this PR, meaning that I can't really make progress on zig-bootstrap. It's some obscure issue in locale code, and I don't feel particularly motivated to investigate that nightmare fuel right now.

[alexrp]

It's some obscure issue in locale code, and I don't feel particularly motivated to investigate that nightmare fuel right now.

Ok well, I did, and apparently when the _LIBCPP_LOCALE__L_EXTENSIONS macro was removed, no NetBSD code path was added, so it ends up including just __locale_dir/locale_base_api/bsd_locale_fallbacks.h which it seems like it probably shouldn't...?

In file included from /home/alexrp/Source/ziglang/zig-bootstrap/out/host/lib/zig/libcxx/src/iostream.cpp:9:
    In file included from /home/alexrp/Source/ziglang/zig-bootstrap/out/host/lib/zig/libcxx/src/std_stream.h:14:
    In file included from /home/alexrp/Source/ziglang/zig-bootstrap/out/host/lib/zig/libcxx/include/__locale:14:
    In file included from /home/alexrp/Source/ziglang/zig-bootstrap/out/host/lib/zig/libcxx/include/__locale_dir/locale_base_api.h:138:
    /home/alexrp/Source/ziglang/zig-bootstrap/out/host/lib/zig/libcxx/include/__locale_dir/locale_base_api/bsd_locale_fallbacks.h:32:72: error: no member named 'uselocale' in the global namespace
       32 |   _LIBCPP_HIDE_FROM_ABI __locale_guard(locale_t& __loc) : __old_loc_(::uselocale(__loc)) {}
          |                                                                      ~~^
    /home/alexrp/Source/ziglang/zig-bootstrap/out/host/lib/zig/libcxx/include/__locale_dir/locale_base_api/bsd_locale_fallbacks.h:36:9: error: no type named 'uselocale' in the global namespace
       36 |       ::uselocale(__old_loc_);
          |       ~~^

cc @brad0 I've seen you do some NetBSD work upstream in LLVM, maybe you'd be interested in this.

[alexrp]

@mikdusan has verified that a cross-compiled program works on NetBSD. This should be ready for review.

As mentioned above, there's still more work to be done in libc++ for zig-bootstrap to work for NetBSD, however.