asm/syscall.h: add syscall_get_arch

Adds a stub for a function that will return the AUDIT_ARCH_* value
appropriate to the supplied task based on the system call convention.

For audit's use, the value can generally be hard-coded at the
audit-site.  However, for other functionality not inlined into syscall
entry/exit, this makes that information available.  seccomp_filter is
the first planned consumer and, as such, the comment indicates a tie to
CONFIG_HAVE_ARCH_SECCOMP_FILTER.

Suggested-by: Roland McGrath <mcgrathr@chromium.org>
Signed-off-by: Will Drewry <wad@chromium.org>
Acked-by: Serge Hallyn <serge.hallyn@canonical.com>
Acked-by: Eric Paris <eparis@redhat.com>

v18: comment and change reword and rebase.
v14: rebase/nochanges
v13: rebase on to 88ebdda6159ffc15699f204c33feb3e431bf9bdc
v12: rebase on to linux-next
v11: fixed improper return type
v10: introduced