std.Thread: make Id smaller where possible

lib/std/Thread.zig

@@ -255,8 +255,19 @@ pub fn getName(self: Thread, buffer_ptr: *[max_name_len:0]u8) GetNameError!?[]co
     return error.Unsupported;
 }
 
-/// Represents a unique ID per thread.
-pub const Id = u64;
+/// Represents an ID per thread guaranteed to be unique only within a process.
+pub const Id = switch (target.os.tag) {
+    .linux,
+    .dragonfly,
+    .netbsd,
+    .freebsd,
+    .openbsd,
+    .haiku,
+    => u32,
+    .macos, .ios, .watchos, .tvos => u64,
+    .windows => os.windows.DWORD,
+    else => usize,
+};
 
 /// Returns the platform ID of the callers thread.
 /// Attempts to use thread locals and avoid syscalls when possible.
@@ -431,7 +442,7 @@ fn callFn(comptime f: anytype, args: anytype) switch (Impl) {
 const UnsupportedImpl = struct {
     pub const ThreadHandle = void;
 
-    fn getCurrentId() u64 {
+    fn getCurrentId() usize {
         return unsupported({});
     }
 
@@ -466,7 +477,7 @@ const WindowsThreadImpl = struct {
 
     pub const ThreadHandle = windows.HANDLE;
 
-    fn getCurrentId() u64 {
+    fn getCurrentId() windows.DWORD {
         return windows.kernel32.GetCurrentThreadId();
     }