[Support] Don't check XCR0 when detecting avx512 on Darwin.

Darwin lazily saves the AVX512 context on first use [1]: instead of checking
that it already does to figure out if the OS supports AVX512, trust that
the kernel will do the right thing and always assume the context save
support is available.

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

Reviewers: ab, RKSimon, craig.topper

Reviewed By: craig.topper

Differential Revision: https://reviews.llvm.org/D70453