target: support scalable vectors on arm64ec

[davidtwco]

This architecture is also AArch64, just with a different ABI, and so supports scalable vectors too.

This is part of what is becoming a series of small PRs which fix CI failures in rust-lang/stdarch#2071 once they reach nightly.

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[taiki-e]

supports scalable vectors too.

AFAIK, arm64ec doesn't support SVE: #131332 (comment).

[Amanieu]

I can confirm that arm64ec doesn't support SVE. The OS only saves the low 128 bits of vector registers, which means that the upper bits could be clobbered at any point from random interrupts. Most likely the OS just disables SVE instructions entirely and makes them unavailable to user mode.

[davidtwco]

Oh, I didn't know that. I'll close this and adjust the cfgs on the stdarch side, thanks.