linux: omit getauxval from binary if not needed

[wooster0]

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.

[wooster0]

I now realize that this check would essentially have to be "if std.os.linux.getauxval was analyzed" or is potentially going to be called somewhere. That makes this a lot harder.

[wooster0]

This should probably be done through an std option. Not doing this for now so just ignore this.

[rohlem]

I think the extern_getauxval and in this if should be superfluous, because it's already implied by the if (extern_getauxval) around the containing struct.
If that branch isn't taken, the compiler should never analyze this comptime block inside of it.

[wooster0]

Good point