Enable vgdb ptrace invoker for aarch64.

This only works in non-bi arch mode. If ever aarch64+arm
are compiled bi-arch, then some more work is needed to have
a 64 bits vgdb able to ptrace invoke a 32 bits valgrind.

Note also that PTRACE_GETREGSET is defined on other platforms
(e.g. ppc64 fedora 18 defines it), but it is not used on
these platforms, as again, PTRACE_GETREGSET implies some
work for bi-arch to work properly.
So, on all platforms except arm64, we use PTRACE_GETREGS
or PTRACE_PEEKUSER.



git-svn-id: svn://svn.valgrind.org/valgrind/trunk@13981 a5019735-40e9-0310-863c-91ae7b9d1cf9