arch-power: Implement mcrxr

arch-power: Implement mcrxr

Implement the mcrxr instruction (Move to Condition Register from XER
X-form) as defined on p.132 of the green-cloth book:
The contents of XER<0:3> are copied into the Condition Register field
designated by BF.  XER<0:3> are set to zero.

Change-Id: I82ae3d98e1eaf9182e90c0c86afe0f13d4a052e4
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/37295
Reviewed-by: Gabe Black <gabe.black@gmail.com>
Reviewed-by: Boris Shingarov <shingarov@gmail.com>
Maintainer: Gabe Black <gabe.black@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>