sparc: properly conditionalize use of TIF_32BIT

After merging the audit tree, today's linux-next build (sparc defconfig)
failed like this:

In file included from include/linux/audit.h:29:0,
                 from mm/mmap.c:33:
arch/sparc/include/asm/syscall.h: In function 'syscall_get_arch':
arch/sparc/include/asm/syscall.h:131:9: error: 'TIF_32BIT' undeclared (first use in this function)
arch/sparc/include/asm/syscall.h:131:9: note: each undeclared identifier is reported only once for each function it appears in

And many more ...

Caused by commit 374c0c054122 ("ARCH: AUDIT: implement syscall_get_arch
for all arches").

This patch wraps the usage of TIF_32BIT in:
   if defined(__sparc__) && defined(__arch64__)
Which solves the build problem.

Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>
Acked-by: Richard Guy Briggs <rgb@redhat.com>
Signed-off-by: Eric Paris <eparis@redhat.com>