x86_64: enable struct field reordering

The blocker for enabling this feature was my need to debug the emitted
assembly without debug info and having to manually inspect memory to
determine struct contents.  However, we now have debug info!

    (lldb) v -L foo bar
    0x00007fffffffda20: (repro.repro.Foo) foo = {
    0x00007fffffffda24:   .x = 12
    0x00007fffffffda20:   .y = 34
    }
    0x00007fffffffda28: (repro.repro.Bar) bar = {
    0x00007fffffffda28:   .x = 56
    0x00007fffffffda2c:   .y = 78
    }

Updates #21530

Author: jacobly0

src/target.zig

@@ -720,7 +720,7 @@ pub inline fn backendSupportsFeature(backend: std.builtin.CompilerBackend, compt
             else => false,
         },
         .field_reordering => switch (backend) {
-            .stage2_c, .stage2_llvm => true,
+            .stage2_c, .stage2_llvm, .stage2_x86_64 => true,
             else => false,
         },
         .safety_checked_instructions => switch (backend) {