x86: spell vpxor %zmm0 as vpxord

[rlibby]

Summary

Fix gcc/gas amd64 & i386 build after r347566.

Test Plan

make buildkernel

Note, this _does_ change the assembly with the clang build from

ffffffff8107f26b: 62 f1 fd 48 ef c0 vpxorq %zmm0, %zmm0, %zmm0
to
ffffffff8107f26b: 62 f1 7d 48 ef c0 vpxord %zmm0, %zmm0, %zmm0

I.e., it was assembly with vpxorq, but now uses d. The assembly goes on
to use d instructions, so I thought that was the intention, but we can
choose q here instead if desired.

[kib]

I suspect that vpxord/vpxorq difference does not matter. If I have an opportunity, I will ask.