Remove Support for Android NDKs before r18
To get rid of the hacky GCC support in platform_macros.GNU, ensure that
all the NDKs we support have >=C++11 by default, and to be able to
default to using lld as recommended by Google.
Also:
- Removed `ANDROID_ARCH` make variable.
- Replaced `ANDROID_ABI` with `android_abi` make variables to match
other make variables, but keep `ANDROID_ABI` as an alias of
`android_abi` for compatibility.
- Will no longer default to 32-bit ARM. `android_abi` or `ANDROID_ABI`
are now required.
- Update ACE-INSTALL.html for all the changes so far