Fix CPU flags in psydac/api/settings.py

[campospinto]

Clean up platform-specific flags for GCC compilers. In particular, do not use the plausible but inexistent flag -mcpu=apple-m4 on Apple M4 computers (which use ARM chips). Instead, always use -march=native, and additionally -mavx on x86_64 architectures. Further, do not use -mtune=native as this is not always supported. These changes fix #568

[codacy-production]

Coverage summary from Codacy

See diff coverage on Codacy

Coverage variation	Diff coverage
✅ -0.01%	✅ 50.00%

Coverage variation details

	Coverable lines	Covered lines	Coverage
Common ancestor commit (4f78a33)	31801	19597	61.62%
Head commit (77025d1)	63576 (+31775)	39174 (+19577)	61.62% (-0.01%)

Coverage variation is the difference between the coverage for the head and common ancestor commits of the pull request branch: <coverage of head commit> - <coverage of common ancestor commit>

Diff coverage details

	Coverable lines	Covered lines	Diff coverage
Pull request (#569)	2	1	50.00%

Diff coverage is the percentage of lines that are covered by tests out of the coverable lines that the pull request added or modified: <covered lines added or modified>/<coverable lines added or modified> * 100%

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[yguclu]

Thanks again for this PR!

This looks good to me now, I just have two minor comments