ptrace: syscall-exit && stepping fixes

Changes: added x86 maintainers + linux-arch, otherwise unchanged.

Currently PTRACE_SINGLESTEP after syscall_exit stop is not consistent across different machines, and we have different problems depending on arch/.

As Roland suggests, we should move the arch-dependent logic into the tracehook_report_syscall_exit() and unify/fix the behaviour.

Oleg.