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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 issue 28046 and the patch fixes also an oversight made in this issue in Makefile.pre.in.
Also tested with the patch in issue 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 issue 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 issue 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 #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 #28849: Merge 3.6.
https://hg.python.org/cpython/rev/40e8b39199da 

New changeset ab3d870aa3bc by Xavier de Gaye in branch '3.6':
Issue #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 #28849: Merge 3.6.
https://hg.python.org/cpython/rev/0ff181ca7558