UM: implement syscall_get_arch()

This patch defines syscall_get_arch() for the um platform.  It adds a
new syscall.h header file to define this.  It copies the HOST_AUDIT_ARCH
definition from ptrace.h.  (that definition will be removed when we
switch audit to use this new syscall_get_arch() function)

Based-on-patch-by: Richard Briggs <rgb@redhat.com>
Signed-off-by: Eric Paris <eparis@redhat.com>
Cc: user-mode-linux-devel@lists.sourceforge.net