TST, BUILD: Add a native x86 baseline build running on ubuntu-20.04

[seiko2plus]

Add a native x86 baseline build running on ubuntu-20.04

to build and test NumPy against the maximum supported CPU features
by the compiler and the running machine

native baseline build will allow us to test pull-requests like #17958,
that doesn't use runtime dispatching.

[seiko2plus]

/cc, @mattip

[mattip]

This new run on ubuntu 20.04 uses the system python3.8 and cpu features

NumPy CPU features: SSE SSE2 SSE3 SSSE3 SSE41 POPCNT SSE42 AVX F16C FMA3 AVX2 AVX512F \
AVX512CD AVX512_SKX

This is different from the github actions python3.8 run which is using cpu features and dispatching

NumPy CPU features: SSE SSE2 SSE3 SSSE3* SSE41* POPCNT* SSE42* AVX* F16C* FMA3* AVX2* AVX512F* \
AVX512CD* AVX512_KNL? AVX512_KNM? AVX512_SKX* AVX512_CNL?

Is there any concern that as azure changes its hardware we might see a different set of baseline features chosen? Thinking about it more, I guess if they change hardware, we could also have problems with dispatching, so this is no more "dangerous" than the current setup.

[mattip]

The only concern I have is the hidden cost of adding an additional CI run, we may want to limit our use of CI resources at some point.

[seiko2plus]

Is there any concern that as azure changes its hardware we might see a different set of baseline features chosen?

Yes, that can normaly happens for example if the running machine only supports AVX2 FMA3 the output will be:

NumPy CPU features: SSE SSE2 SSE3 SSSE3 SSE41 POPCNT SSE42 AVX F16C FMA3 AVX2

Thinking about it more, I guess if they change hardware, we could also have problems with dispatching

No we don't, --cpu-baseline=native is based on the compiler native operation which only enables the CPU features
that supported by the local machine, please take a look at x86-Options for more clearfication.

The only concern I have is the hidden cost of adding an additional CI run, we may want to limit our use of CI resources at some point.

But we still need to add another build for baseline_avx2 since its seems most of builders run on avx512 machine.

[mattip]

Thanks @seiko2plus