Deal with macOS not enabling AVX512 state in XCR0

All Mac Pros launched since 2019 have AVX512 support (CLX processor) but
the Darwin kernel does not enable the state bits in the XCR0 register
for all processes by default. Instead, it traps the #UD exception and
decodes the instruction to find out if it was an AVX512 instruction. If
so, it adds memory to the thread's context switch space. See [1].

Good solution, but forces us to have OS-specific code to determine if
the OS really supports AVX512. No other OS appears to require this. For
future features (namely, Intel Advanced Matrix Extensions), there's an
official way of implementing this, via the eXtended Feature Disable
register.

Qt has no AVX512 code yet, so this is just being future-proof. Apple has
yet to announce when or if their Mac Pros will switch from Intel Xeon to
ARM.

[1] https://github.com/apple/darwin-xnu/blob/xnu-4903.221.2/osfmk/i386/fpu.c#L174

Change-Id: Ieffc3453b88c4517a1dbfffd162175ead343e622
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@qt.io>

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