Add support for Supervisor Mode Access Prevention (SMAP)

[Martin Decky]

Supervisor Mode Access Prevention (SMAP) is a feature of the Haswell microarchitecture (and later) that prevents the kernel from accessing user space memory. This basically limits the attack surface on the kernel because the kernel can decide to access the user space memory only in certain, tightly controlled situations (e.g. in copy_from_uspace(), copy_to_uspace(), etc.).