Android: Add CMAKE_<LANG>_ANDROID_TOOLCHAIN_MACHINE

Expose the binutils' machine name (typically used as a prefix on the
tool names) publicly.  This is expected to match the `gcc -dumpmachine`
value.

Suggested-by: Ruslan Baratov <ruslan_baratov@yahoo.com>
Status Job ID Name Coverage
  External
passed kwrobot-branch-check