Android is not a multiarch system and the unix build system can cross compile non-multiarch systems as well as multiarch systems. This is a follow up of the decisions implemented in bpo-28046 and the patch fixes also an oversight made in this issue in Makefile.pre.in.

Also tested with the patch in bpo-28833 that implements cross compilation of third-party extension modules.
Please run autoconf after installing the patch.

Actually all UNIX-like platforms now have non-empty sys.implementation._multiarch now (msg273875). An idea is renaming it to something like target_architecture to make things clearer. It will be a triplet on all system, whether cross-built or not.

An idea is renaming it to something like target_architecture
to make things clearer

Yes, that was my intent, and I think it is the right solution. using $(MACHDEP)_$(MULTIARCH) in the sysconfigdata name and probably in other places was not a good idea (introduced by the platdir removal).

There's no reason why this target_architecture could be used by any unix like platform.

The change made by the patch in Makefile.pre.in handles the removing of the dangling underscore in the patch of bpo-28833, it is not needed.
android_multiarch_2.patch is simpler: the change in Makefile.pre.in has been removed and test_triplet_in_ext_suffix is now skipped when sys.implementation does not have the _multiarch attribute instead of only for android as previously.

The MULTIARCH triplet:

• is used by the build system in the suffix of the name of the _sysconfigdata private module, it may have any value (including being empty) when the platform is non-multiarch - for example cross-compiling for an Android emulator with the 'foo-bar-foobar' MULTIARCH triplet works just fine (without the patch)
• is not used by the interpreter
• the Python user may not use it (sys.implementation_multiarch is private)
• is not used in the standard library, except by the sysconfig modules to get the name of the _sysconfigdata module (and it may be empty)
• is not needed any more to cross-compile non-multiarch platforms (even without the patch) since bpo-28046, where platform-specific directories have been removed

On Android the MULTIARCH triplet adds the burden of having to maintain the list of Android processors and abis in configure.ac through a set of complex conditionals (see below [1]) whose correctness is uncertain and not verified. This is useless and removed since it is not needed anymore.

[1] The Android set of conditionals in configure.ac:

#if defined(__ANDROID__)
# if defined(__x86_64__) && defined(__LP64__)
        x86_64-linux-android
# elif defined(__i386__)
        i686-linux-android
# elif defined(__aarch64__) && defined(__AARCH64EL__)
#  if defined(__ILP32__)
        aarch64_ilp32-linux-android
#  else
        aarch64-linux-android
#  endif
# elif defined(__ARM_EABI__) && defined(__ARMEL__)
        arm-linux-androideabi
# elif defined(__mips_hard_float) && defined(_MIPSEL)
#  if _MIPS_SIM == _ABIO32
        mipsel-linux-android
#  elif _MIPS_SIM == _ABI64
        mips64el-linux-android
#  else
#   error unknown platform triplet
#  endif
# else
#   error unknown platform triplet
# endif

New changeset b3ba41bf92c7 by Xavier de Gaye in branch '3.6':
Issue bpo-28849: Do not define sys.implementation._multiarch on Android.
https://hg.python.org/cpython/rev/b3ba41bf92c7

New changeset 40e8b39199da by Xavier de Gaye in branch 'default':
Issue bpo-28849: Merge 3.6.
https://hg.python.org/cpython/rev/40e8b39199da

New changeset ab3d870aa3bc by Xavier de Gaye in branch '3.6':
Issue bpo-28849: Skip test_sysconfig.test_triplet_in_ext_suffix on non linux platforms.
https://hg.python.org/cpython/rev/ab3d870aa3bc

New changeset 0ff181ca7558 by Xavier de Gaye in branch 'default':
Issue bpo-28849: Merge 3.6.
https://hg.python.org/cpython/rev/0ff181ca7558