arch: Detect all Apple-silicon chips as AppleArm

[ggorman]

Problem

get_platform() recognises Apple silicon only via 'm1' in brand (devito/arch/archinfo.py). A brand string such as "Apple M2 Pro" or "Apple M4 Max" falls through to ANYCPU, so @skipif('cpu64-arm') does not fire. tests/test_autotuner.py::test_nested_nthreads then runs with platform='knl7210', the generated code includes xmmintrin.h/pmmintrin.h, and Homebrew clang on arm64 rejects it:

xmmintrin.h:14:2: error: "This header is only meant to be used on x86 and x64 architecture"

Which chip a macos-26-arm64 runner gets is a lottery, so the same code is green on one run and red on the next. Seen on #3047: that run had zero Arm doesn't support x86-specific instructions skips across the suite, while the main run the day before (same image, same LLVM 23.1.0) skipped the test.

Fix

Map any apple m<N> brand to the registered AppleArm of that generation, falling back to the newest registered one for chips not yet listed; AppleArm.march derives the actual -mcpu at runtime from sysctl. One branch replaced in get_platform.

Test

tests/test_arch.py::test_detect_apple_silicon, parametrised over "Apple M1", "Apple M2 Pro", "Apple M3 Max", "Apple M4 Max": monkeypatches get_cpu_info and asserts get_platform yields an AppleArm.

• Negative control: with archinfo.py reverted, the M2/M3/M4 cases fail and only M1 passes; with the change all four pass.

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 84.23%. Comparing base (2707ac9) to head (bc74301).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3048      +/-   ##
==========================================
- Coverage   84.23%   84.23%   -0.01%     
==========================================
  Files         258      258              
  Lines       57070    57071       +1     
  Branches     4874     4874              
==========================================
- Hits        48072    48071       -1     
- Misses       8170     8172       +2     
  Partials      828      828              

Flag	Coverage Δ
pytest-gpu-aomp-amdgpuX	68.51% <0.00%> (+<0.01%)	⬆️
pytest-gpu-gcc-	78.99% <0.00%> (-0.01%)	⬇️
pytest-gpu-icx-	78.91% <0.00%> (-0.01%)	⬇️
pytest-gpu-nvc-nvidiaX	69.08% <0.00%> (-0.01%)	⬇️

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[EdCaunt]

Not unreasonable, but lots of noise in here.