arm64: ptrace: add XZR-safe regs accessors

In A64, XZR and the SP share the same encoding (31), and whether an
instruction accesses XZR or SP for a particular register parameter
depends on the definition of the instruction.

We store the SP in pt_regs::regs[31], and thus when emulating
instructions, we must be careful to not erroneously read from or write
back to the saved SP. Unfortunately, we often fail to be this careful.

In all cases, instructions using a transfer register parameter Xt use
this to refer to XZR rather than SP. This patch adds helpers so that we
can more easily and consistently handle these cases.

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