Move feature descriptions out of std.Target.Cpu.Feature

[squeek502]

I happened to notice that there were a bunch of these description strings in the data section of compiled resinator binaries. It turns out they were coming from Aro because Aro uses std.Target. Aro does not actually use the description strings, though, so in both Aro and resinator, those strings are wasted space in the binary.

So, this PR allows users of std.Target to not include all the description strings in their program if they don't actually use them.

Counting all the bytes of the description strings after they were separated out using this code:

comptime {
    @setEvalBranchQuota(10000000);
    var total_byte_count: usize = 0;
    for (std.enums.values(Arch)) |arch| {
        const descs = allFeaturesDescList(arch);
        for (descs) |desc| {
            total_byte_count += desc.len;
        }
    }
    @compileLog(total_byte_count);
}

gives a total size of 146898 bytes or ~143 KiB. However, I have not seen quite that much saved in compiled binaries in practice. A test program like this:

const std = @import("std");

pub fn main() !void {
    const target = try std.zig.system.resolveTargetQuery(.{});
    std.debug.print("{any}\n", .{target});
}

compiled in ReleaseSmall with -fstrip on Windows gives these results:

• Before: 321024 bytes (313.5 KiB)
• After: 219648 bytes (214.5 KiB)
• Delta: 101376 bytes ( 99.0 KiB)

Side note: the use of std.Target still increases the size of the binary a lot; for comparison, a simple hello world compiled in ReleaseSmall is 5 KiB

Note: The spirv features were not intended to be changed, but I'm not sure what exact versions of the Headers/Registry were used to generate the current Target/spirv.zig. I used commits around the time of the last Target/spirv.zig update in May 2021 (98dc8eb); in particular, I used commits 60af2c93c46294a2bc9758889a90d935b6f9325f for SPIRV-Registry and ba29b3f59633836c6bb160b951007c8fc3842dee for SPIRV-Headers. Newer Registry/Headers versions were not used because (1) there are a lot more features, more than the current Feature.Set.needed_bit_count and (2) the current update_spirv_features.zig cannot handle some of the new files, e.g. https://raw.githubusercontent.com/KhronosGroup/SPIRV-Registry/main/extensions/EXT/SPV_EXT_relaxed_printf_string_address_space.asciidoc

cc @Snektron

[Snektron]

Note: The spirv features were not intended to be changed, but I'm not sure what exact versions of the Headers/Registry were used to generate the current Target/spirv.zig

Oof, that is a good question. Currently its not terribly important, this part needs a little bit more thinking anyway. I'm fine with the current changes.

[andrewrk]

Sorry but I want to instead solve this with #21010 which is a low-priority enhancement.

[squeek502]

Would it suffice to add in some // TODO: https://github.com/ziglang/zig/issues/21010 comments to get this merged?