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 <d3e8afea784ab4a50a2dbf9149d2eaa8478e1cbc@gmail.com>
Reviewed-by: Boris Shingarov <9e9a86074d235a5ecff50c3f2485c8b344969b31@gmail.com>
Maintainer: Gabe Black <d3e8afea784ab4a50a2dbf9149d2eaa8478e1cbc@gmail.com>
Tested-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>

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