remove redundant `cpu`/`os`/`abi`/`object_format` declarations from `@import("builtin")`

[mlugg]

Right now, details of the compilation target can be accessed through @import("builtin") in multiple ways:

• builtin.target.cpu and builtin.cpu
• builtin.target.os and builtin.os
• builtin.target.abi and builtin.abi
• builtin.target.ofmt and builtin.object_format

These are guaranteed to always be equivalent -- target is actually just defined like this:

pub const target: std.Target = .{
    .cpu = cpu,
    .os = os,
    .abi = abi,
    .ofmt = object_format,
    .dynamic_linker = .init("/lib64/ld-linux-x86-64.so.2"), // this line varies per target
};

As such, having both around is entirely redundant, and has no benefit. This is a violation of "Only one obvious way to do things".

Since builtin.target bundles these values together in a logical way, it is the more useful declaration (it is easier and more concise to get a field from target than it is to reconstruct target from its fields). Therefore, this is a proposal to remove the following declarations from the generated builtin.zig source code:

• cpu
• os
• abi
• object_format

These declarations should be marked as deprecated for one release cycle, and then entirely removed in a following one. I expect that the current definitions of cpu, os, abi, and object_format will just be inlined into the definition of target.