I've looked again at it and it is actually less problematic that I
first thought but I still see the following two cases:

1. vfork seems to not set any TIF_ flags so a ptracer setting regs
while a vforking task is stopped in ptrace_notify called from vfork
would result in clobbered %ecx and %edx.

2. A ptracer could use %ecx or %edx to pass information to signal
handlers and this would not work with the current [rt_]sigsuspend.

These only need setting TIF_IRET on ptrace setregs though.

There is also the very small advantage of being able to hardcode
SYSENTER_RETURN as the return eip for sysexit if TIF_IRET is set in
all the 3 places.