Adding support to not native x86 systems for external libraries

[mghibaudi]

• Filtering of x86 specific directive at both Makefile and setup.py level. Former needed for libraries that integrate Stim at the binary level (e.g. PyMatching).
• Replacing sys with platform to gain access to additional information on the OS and CPU architecture.
• Wrapped up with a function the extensions that shall be built for a specific architecture.

[Strilanc]

Is there a way to test these working, from github actions, so that I don't accidentally break them given that I'm running on x86 machines?

[Strilanc]

The new muxing logic is failing on widows and osx, as you can see in the CI tests.

[mghibaudi]

Thanks @Strilanc for the quick review - enabled CI on the fork and debugged the issues (i386, i686 were missing - thus Win and Mac failures). Current version passes cleanly on my forkd' main.

[mghibaudi]

@Strilanc - just checking did you manage to have a look at the latest changes? they do fix the previous error and pass in CI (on the forked branch).