ARMv8-nofp support

[jharris-intel]

Is this request related to a missing driver support for a particular hardware platform, SoC or board? Please describe.

ARMv8 has the following to say about FPU/SIMD support:

ARMv8 can support the following levels of support for floating-point and Advanced SIMD instructions:
• Full floating-point and SIMD support without exception trapping.
• Full floating-point and SIMD support with exception trapping.
• No floating-point or SIMD support. This option is licensed only for implementations targeting specialised
markets.

("specialized markets" here meaning "embedded systems" and suchlike... which as you can no doubt tell from this ticket includes the market I am in :-) )

I would like support for ARMv8 with no floating-point or SIMD, as described above.

Describe why you are asking for this support?

I have a few fairly minor patches to enable ARMv8-nofp support in Zephyr, and would like to upstream them. (I would directly create a PR, although this is somewhat blocked behind the chosen direction of #32192). I would much prefer if ARMv8-nofp support was upstreamed, as that would make maintenance easier. (It's much better in general to try to upstream such things.)

I would like to maintain support for this feature, although I cannot guarantee time on this in perpetuity. (It would be fully acceptable if support was dropped should I stop maintaining this without finding someone to pass it off to.)

Additional context

Note that you can fairly easily test this feature on an ARMv8 implementation that does have an FPU by setting CPTR_EL3.TFP to 1 in early boot in EL3 (or CPTR_EL2.TFP in EL2). This will result in any FPU operations trapping, which can confirm that no floating-point operations were used.

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