link.Elf: Don't emit malformed archive headers

[castholm]

Closes #25129

Brought to my attention by a post on Ziggit. The self-hosted ELF linker currently initializes all archive header fields to all spaces without any digits, which ld.lld doesn't like.

Tested by compiling

//! By convention, root.zig is the root source file when making a library. If
//! you are making an executable, the convention is to delete this file and
//! start with main.zig instead.
const std = @import("std");
const testing = std.testing;

export fn add_one(number: i64) i64 {
    return number + 1;
}

test "basic add functionality" {
    try testing.expect(add_one(3) == 4);
}

(from #25129 (comment)) using zig build-lib root.zig -target x86_64-linux-gnu -fno-llvm. Before/after:

 Offset(h) 00 01 02 03 04 05 06 07 08 09 0a 0b 0c 0d 0e 0f
 
 00000000  21 3c 61 72 63 68 3e 0a 2f 53 59 4d 36 34 2f 20  !<arch>./SYM64/ 
-00000010  20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20                  
+00000010  20 20 20 20 20 20 20 20 30 20 20 20 20 20 20 20          0       
-00000020  20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20                  
+00000020  20 20 20 20 30 20 20 20 20 20 30 20 20 20 20 20      0     0     
-00000030  20 20 20 20 20 20 20 20 32 34 20 20 20 20 20 20          24      
+00000030  30 20 20 20 20 20 20 20 32 34 20 20 20 20 20 20  0       24      
 00000040  20 20 60 0a 00 00 00 00 00 00 00 01 00 00 00 00    `.............
 00000050  00 00 00 5c 61 64 64 5f 6f 6e 65 00 72 6f 6f 74  ...\add_one.root
-00000060  2e 6f 2f 20 20 20 20 20 20 20 20 20 20 20 20 20  .o/             
+00000060  2e 6f 2f 20 20 20 20 20 20 20 20 20 30 20 20 20  .o/         0   
-00000070  20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20                  
+00000070  20 20 20 20 20 20 20 20 30 20 20 20 20 20 30 20          0     0 
-00000080  20 20 20 20 20 20 20 20 20 20 20 20 38 38 34 33              8843
+00000080  20 20 20 20 30 20 20 20 20 20 20 20 37 36 38 30      0       7680
-00000090  31 32 30 20 20 20 60 0a 7f 45 4c 46 02 01 01 00  120   `..ELF....
+00000090  35 32 38 20 20 20 60 0a 7f 45 4c 46 02 01 01 00  528   `..ELF....

Side note: There are three different archive definitions in the code base, std.elf.ar_hdr, link.MachO.Archive.ar_hdr and link.Wasm.Archive.Header, which each do things in subtly different ways. Maybe it would be useful to consolidate them into std.ar or something like that?

[squeek502]

Thanks!