Move EXECUTORCH_PAL_DEFAULT to default preset (#10798)

### Summary
TSIA

### Test plan

CI + 

```
$ cmake --preset macos-arm64 && cmake --build cmake-out --parallel
```
```
$ cmake -DEXECUTORCH_PAL_DEFAULT=fake --preset macos-arm64

CMake Error at tools/cmake/preset/default.cmake:48 (message):
  PAL default implementation (fake) file not found:
  /Users/jathu/executorch/runtime/platform/default/fake.cpp
Call Stack (most recent call first):
  CMakeLists.txt:53 (include)
```

cc @larryliu0820