This register is visible to software executing at EL0 but not writeable.
Storing it in the VCPU context required custom save/restore handling as
it had to be explicitly handled when switching from a VCPU thread to a
non-VCPU thread so that it didn't become a channel. It is possible to
now update this register via seL4_TCB_WriteRegisters for software
executing at EL0.
This also fixes a potential bug where if a vcpu-thread is switched for a
non-vcpu-thread and then switched to a different vcpu-thread the
original vcpu-thread's copy of this register will get set to 0.
8234026c1f