PTRACE_SYSEMU behavior difference on arm64

[Keno Fischer]

The behavior of PTRACE_SYSEMU on arm64
appears to differ substantially from that of x86 and powerpc
(the other two architectures on which this feature is implemented).
In particular, after PTRACE_SYSEMU the syscall will always
be skipped on x86 and powerpc, but executed on arm64 unless
the syscall-entry stop was again continued using PTRACE_SYSEMU.
The skipping behavior is also documented in the manpage,
so I suspect this may just be a bug (the skipping behavior
makes sense to me and is what I would expect).
The reason this happens is that `syscall_trace_enter`
re-checks TIF_SYSCALL_EMU after the ptrace stop, but at that
point it may have already been superseded by a new ptrace
request. x86 and power save the original value of the flag,
rather than acting on the new value. I can submit a patch to
fix this, but wanted to check first whether this was intentional.
If it is, I can fix the man page instead.

Keno