mach: add a standard way to access platform specific structs

This is done by adding platform.zig which is a file providing types like
``CoreType``, ``GpuDriverType`` and ``BackingTimerType``.

src/platform.zig

@@ -0,0 +1,8 @@
+const builtin = @import("builtin");
+
+const Platform = if (builtin.cpu.arch == .wasm32) @import("wasm.zig") else @import("native.zig");
+
+// TODO: verify declarations and its signatures
+pub const CoreType = Platform.Core;
+pub const GpuDriverType = Platform.GpuDriver;
+pub const BackingTimerType = Platform.BackingTimer;