[BUG] The legacy toolchain file incorrectly sets `CMAKE_ANDROID_ARM_MODE`

### Description

`CMAKE_ANDROID_ARM_MODE` should be `ON`/`OFF` per [its doc](https://cmake.org/cmake/help/latest/variable/CMAKE_ANDROID_ARM_MODE.html), but the legacy toolchain file will set it to `ANDROID_ARM_MODE`, which is `thumb`/`arm`, so `CMAKE_ANDROID_ARM_MODE` will always be evaluated as `ON`.
I'm surprised it's broken since the beginning: https://android-review.googlesource.com/c/platform/ndk/+/360029

### I am using a supported NDK

- [x] I have checked and the NDK I'm using is currently supported

### Affected versions

r27, r28, r29

Locate the legacy toolchain file and trace how CMAKE_ANDROID_ARM_MODE is assigned from ANDROID_ARM_MODE. Compare the resulting value with the CMake variable documentation, then verify that the affected NDK versions produce the intended ON/OFF value.

Written by the indexing model from the issue text.