linux: omit getauxval from binary if not needed

[ghost]

Fixes #21522

@rohlem's suggestion in #21522 to move the relevant comptime block into the container namespace used to define getauxval seems to work fine.

$ cat hello_world.zig
const std = @import("std");

pub const std_options = std.Options{ .keep_sigpipe = true };

pub fn main() void {
    std.io.getStdOut().writeAll("hello world\n") catch {};
}
$ zig build-exe hello_world.zig -OReleaseSmall -fsingle-threaded && wc -c hello_world
2016 hello_world
$ zig build-exe hello_world.zig -OReleaseSmall -fsingle-threaded --zig-lib-dir lib && wc -c hello_world
1944 hello_world
$

Before: 2016 bytes. After: 1944 bytes.

Might still not address all cases but I guess a new more specific issue can be opened for that.

[alexrp]

Just for my edification, what's the technical reason for needing this @export?

[ghost]

Honestly, I don't know. Maybe it can be removed in a different PR.
I'm not really easily able to learn much more about it right now because there hasn't been a new master build for download in over a week.

[alexrp]

I went digging through the history. The reason is that we only want a single getauxval implementation to be used at runtime (only the one that has elf_aux_maybe set from the start code is actually functional), so we're using weak linkage to accomplish that.

This is mainly relevant when linking multiple Zig compilation units together.