[ag] eliminate hacky workaround for 64-bit builds

* HACK: replace all toolchain references to `linux-x86_64` with `linux-x86`
* FIX: provide dependency on `file` package that will allow `ndk-build` to
  detect 32-bit userland, making this

* side-effects:
  * group sdk/env vars with installation code that uses them
  * add explanatory note about why we use outdated version of `android-ndk`

------------------

*Explanation:*

More careful analysis of the meaning of the word `file` in this (subtle!) error message:

```shell
/opt/android-sdk-linux/android-ndk-r12b/build/ndk-build: 143: /opt/android-sdk-linux/android-ndk-r12b/build/ndk-build: file: not found
```

led to inspecting line 143 of `ndk-build`, revealing an undefined variable
called `file` that would allow `ndk-build` to detect a 32-bit userland:

```shell
file -L "$SHELL" | grep -q "x86[_-]64"
```
Thus the error messsage was *not* trying to tell us that a file could
not be found, but that the program called `file` could not be
found. FUN! :)

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