selftests/powerpc: Add a test of wild bctr

This tests that a bctr (Branch to counter and link), ie. a function
call, to a wildly out-of-bounds address is handled correctly.

Some old kernel versions didn't handle it correctly, see eg:

  "powerpc/slb: Force a full SLB flush when we insert for a bad EA"
  https://lists.ozlabs.org/pipermail/linuxppc-dev/2017-April/157397.html

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

tools/testing/selftests/powerpc/include/reg.h

@@ -17,6 +17,7 @@
 				    : "memory")
 
 #define mb()		asm volatile("sync" : : : "memory");
+#define barrier()	asm volatile("" : : : "memory");
 
 #define SPRN_MMCR2     769
 #define SPRN_MMCRA     770