arm64: ptrace: Override SPSR.SS when single-stepping is enabled

Luis reports that, when reverse debugging with GDB, single-step does not
function as expected on arm64:

  | I've noticed, under very specific conditions, that a PTRACE_SINGLESTEP
  | request by GDB won't execute the underlying instruction. As a consequence,
  | the PC doesn't move, but we return a SIGTRAP just like we would for a
  | regular successful PTRACE_SINGLESTEP request.

The underlying problem is that when the CPU register state is restored
as part of a reverse step, the SPSR.SS bit is cleared and so the hardware
single-step state can transition to the "active-pending" state, causing
an unexpected step exception to be taken immediately if a step operation
is attempted.

In hindsight, we probably shouldn't have exposed SPSR.SS in the pstate
accessible by the GPR regset, but it's a bit late for that now. Instead,
simply prevent userspace from configuring the bit to a value which is
inconsistent with the TIF_SINGLESTEP state for the task being traced.

Cc: <[email protected]>
Cc: Mark Rutland <[email protected]>
Cc: Keno Fischer <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Reported-by: Luis Machado <[email protected]>
Tested-by: Luis Machado <[email protected]>
Signed-off-by: Will Deacon <[email protected]>

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