arch: arc: add support for lock/unlock VPX

Adds support for cooperative locking/unlocking the VPX vector registers.
Provided that all VPX enabled threads use these routines to control
access to the VPX vector registers, it will allow multiple threads to
safely use them without the need for saving/restoring them upon each
context switch.

Signed-off-by: Peter Mitsis <peter.mitsis@intel.com>