Motivation:

A user might want to use a different dynamic library or machine code
depending on the current OS and architecture. Currently, such a user has
to introduce a runtime check in a wrapper method, e.g.

```
@code("4D01C8 4989D9 89D0 0FA2 418900 41895804 41894808 4189500C 4C89CB C3")
private static native void cpuid_amd64_win(int func, int[] out, long out_base_offset);
@code("488D3C11 89F0 4889DE 0FA2 8907 895F04 894F08 89570C 4889F3 C3")
private static native void cpuid_amd64_linux(int func, int[] out, long out_base_offset);

private static final boolean is_windows = System.getProperty("os.name").toLowerCase().contains("windows");

public static void cpuid(int func, int[] out) {
  if (is_windows) {
    cpuid_amd64_win(func, out, UNSAFE.ARRAY_INT_BASE_OFFSET);
  } else {
    cpuid_amd64_linux(func, out, UNSAFE.ARRAY_INT_BASE_OFFSET);
  }
}
```

We could improve the experience by allows a user specify multiple
annotations with the desired OS and architecture combination.

Modifications:

- Added two new enums, `Os` and `Arch`, to represent the current and
  desired OS and architecture.
- Added `AnnotationUtil.findLibrary()` and `findCode()` that looks for
  the annotation that best-matches the expected OS and architecture.
- Updated `Linker` to use `AnnotationUtil` to find the right annotation.
- Updated `CallingConvention` to use `Os.CURRENT` and `Arch.CURRENT` to
  detect the current OS and architecture.
- Added the `CpuId` example that demonstrates this feature.

Result:

A user can now greatly simplify the above example:

```
@code(
  os = Os.WINDOWS,
  arch = Arch.AMD64,
  value = "4989D9 89D0 0FA2 418900 41895804 41894808 4189500C 4C89CB C3"
)
@code(
  os = Os.LINUX,
  arch = Arch.AMD64,
  value = "4889D7 89F0 4889DE 0FA2 8907 895F04 894F08 89570C 4889F3 C3"
)
private static native void cpuid(int func, int[] out);
```

Closes apangin#5