macho: dump linker's state as JSON

Each element of the output JSON has the VM address of the generated
binary nondecreasing (some elements might occupy the same VM address
for example the atom and the relocation might coincide in the address
space).

The generated JSON can be inspected manually or via a preview tool
`zig-snapshots` that I am currently working on and will allow the user
to inspect interactively the state of the linker together with the
positioning of sections, symbols, atoms and relocations within each
snapshot state, and in the future, between snapshots too. This should
allow for quicker debugging of the linker which is nontrivial when
run in the incremental mode.

Note that the state will only be dumped if the compiler is built with
`-Dlink-snapshot` flag on, and then the compiler is passed `--debug-link-snapshot`
flag upon compiling a source/project.

src/Compilation.zig

@@ -757,6 +757,8 @@ pub const InitOptions = struct {
     subsystem: ?std.Target.SubSystem = null,
     /// WASI-only. Type of WASI execution model ("command" or "reactor").
     wasi_exec_model: ?std.builtin.WasiExecModel = null,
+    /// (Zig compiler development) Enable dumping linker's state as JSON.
+    enable_link_snapshots: bool = false,
 };
 
 fn addPackageTableToCacheHash(
@@ -1438,6 +1440,7 @@ pub fn create(gpa: *Allocator, options: InitOptions) !*Compilation {
             .is_test = options.is_test,
             .wasi_exec_model = wasi_exec_model,
             .use_stage1 = use_stage1,
+            .enable_link_snapshots = options.enable_link_snapshots,
         });
         errdefer bin_file.destroy();
         comp.* = .{