arm64/kprobes: consistently handle MRS/MSR with XZR

Now that we have XZR-safe helpers for fiddling with registers, use these
in the arm64 kprobes code rather than open-coding the logic.

Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Will Deacon <will.deacon@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>