arm64: Register modification during syscall entry/exit stop

[Keno Fischer]

Continuing my theme of "weird things I encounter
while trying to use ptrace on arm64", I ran into the
effect of the following code in the syscall entry/exit
reporting:

```
/*
* A scratch register (ip(r12) on AArch32, x7 on AArch64) is
* used to denote syscall entry/exit:
*/
regno = (is_compat_task() ? 12 : 7);
saved_reg = regs->regs[regno];
regs->regs[regno] = dir;
```

This seems very weird to me. I can't think of any
other architecture that does something similar
(other than unicore32 apparently, but the ptrace
support there seems like it might have just been
copied from ARM). I'm able to work around this
in my application, but it adds another stumbling block.
Some examples of things that happen:
- Writes to x7 during syscall exit stops are ignored, so
  if the ptracer tries to emulate a setjmp-type thing, it
  might miss this register (ptracers sometimes like to do
  this to manually serialize execution between different
  threads, puppeteering a single thread of execution
  between different register states).
- Reads from x7 are incorrect, so if the ptracer saves
  a register state and later tries to set it back to the task,
  it may get x7 incorrect, but user space may be expecting
  the register to be preserved (when might this happen? -
  consider a ptracer that wants to modify some syscall
  arguments, it modifies the arguments, restarts the syscall
  but then incurs a signal, so it tries to restore the original
  registers to let userspace deal with the signal without
  being confused - expect signal traps don't ignore x7
  modifications, so x7 may have been unexpectedly
  modified).
- We now have seccomp traps, which kind of look and
  act like syscall-entry traps, but don't have this behavior,
  so it's not particularly reliable for ptracers to use.

Furthermore, it seems unnecessary to me on modern
kernels. We now have PTRACE_GET_SYSCALL_INFO,
which exposes this information without lying to the ptracer
about the tracee's registers.

I understand, we can't just change this, since people may
be relying on it, but I would like to propose adding a ptrace
option (PTRACE_O_ARM_REGSGOOD?) that turns this
behavior off. Now, I don't think we currently have any other
arch-specific ptrace options, so maybe there is a different
option that would be preferable (e.g. could be a different
regset), but I do think it would be good to have a way to
operate on the real x7 value. As I said, I can work around it,
but hopefully I will be able to save a future implementer
some headache.

Keno