The arguments <pstatefield> is a struct that includes two elements,
element reg is special register, elememt enc is pstate field values.
The current code compares two different type values and get a incorrect
result.

The fix follows pstate field to create a systemreg struct,
each system register has a vaule to use in instruction.

Uncomment the msr/mrs cases.

Fixes #21464

Change-Id: I1bb1587ec8548f3e4bd8d5be4d7127bd10d53186
Reviewed-on: https://go-review.googlesource.com/56030Reviewed-by: Cherry Zhang <cherryyz@google.com>
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>