Context switching AVX-512BW 64-bit mask registers unsupported in 32-bit mode.

xref #1312

The kmovq instruction is invalid in 32-bit mode. Currently the only way I see to save and restore mask registers is to use xsave. In the first implementation of AVX-512 context switching, we fallback to using kmovw in 32-bit mode, which will miss the upper 48 bits of each mask register, potentially leaving them subject to being clobbered.

This issue here is a FIXME to track this.