Enable PCID for x86 kernel

seL4 is currently built with PCIDs disabled because QEMU TCG lacks support. This ensured the x86_64_generic and x86_64_generic_vtx configurations remained universally compatible.

Keeping PCIDs disabled leaves x86-64 performance on the table for real silicon.

We should investigate a better build or runtime strategy to enable PCIDs on bare metal while maintaining QEMU TCG compatibility.