RFC-3: Update user context for ARM with thread IDs

Switched appropriate naming conventions.
Was using the aarch64, have switched to aarch64 names.

TIPDRURW -> tpidr_el0
TPIDRURO -> tpidrro_el0
TPIDRPRW -> tpidr_el1

Switch TLS register on aarch32 from TPIDURO (tpidrro_el0) to tpidr_ro so
that it can be written to from user-land.

Thread ID registers tpidr_el0 have been added to the user context for
aarch32 and aarch64.

Only the thread ID that is writeable from EL0 is saved in the TCB and
saved/restored on context switch.

Thread IDs that are only changed within a VM (the read-only thread ID
for exception level 0 and the thread ID for exception level 1) are
stored in the VCPU and saved and stored as part of VM enable/disable.

Thread IDs that are only changed with VMs have been separated out into
hypervisor code.