syscall.c: add ability to set personality based on GETREGSET iov size

syscall.c: add ability to set personality based on GETREGSET iov size

Some architectures (aarch64, s390x) use only PTRACE_GETREGSET interface
and use its size to detect current personality. Let's generalise this
approach and also avoid subtle errors when we get register but forget to
update personality, at least for those architectures.

Note that in order to employ this behaviour, architecture has to use
PTRACE_GETREGSET exclusively (no HAVE_GETREGS_OLD) and should declare
appropriate ARCH_PERSONALITY_*_IOV_SIZE macros.

* syscall.c (get_regs) [ptrace_getregset_or_getregs &&
!HAVE_GETREGS_OLD]: Call update_personality based on the value returned
in the iov_len field by PTRACE_GETREGSET.  Warn once if the returned
iov_len is unknown.