sparc: implement is_32bit_task

We are currently embedding the same check from thread_info.h into
syscall.h thanks to the way syscall_get_arch() was implemented in the
audit tree.  Instead create a new function, is_32bit_task() which is
similar to that found on the powerpc arch.  This simplifies the
syscall.h code and makes the build/Kconfig requirements much easier
to understand.

Signed-off-by: Eric Paris <eparis@redhat.com>
Acked-by: David S. Miller <davem@davemloft.net>
Cc: Stephen Rothwell <sfr@canb.auug.org.au>
Cc: sparclinux@vger.kernel.org