selftests/powerpc: Fix build failure in ebb tests

We use OUTPUT directory as TMPOUT for checking no-pie option.

Since commit f2f02ebd ("kbuild: improve cc-option to clean up all
temporary files") when building powerpc/ from selftests directory, the
OUTPUT directory points to powerpc/pmu/ebb/ and gets removed when
checking for -no-pie option in try-run routine, subsequently build
fails with the following:

  $ make -C powerpc
  ...
  TARGET=ebb; BUILD_TARGET=$OUTPUT/$TARGET; mkdir -p $BUILD_TARGET; make OUTPUT=$BUILD_TARGET -k -C $TARGET all
  make[2]: Entering directory '/home/linux-master/tools/testing/selftests/powerpc/pmu/ebb'
  make[2]: *** No rule to make target 'Makefile'.
  make[2]: Failed to remake makefile 'Makefile'.
  make[2]: *** No rule to make target 'ebb.c', needed by '/home/linux-master/tools/testing/selftests/powerpc/pmu/ebb/reg_access_test'.
  make[2]: *** No rule to make target 'ebb_handler.S', needed by '/home/linux-master/tools/testing/selftests/powerpc/pmu/ebb/reg_access_test'.
  make[2]: *** No rule to make target 'trace.c', needed by '/home/linux-master/tools/testing/selftests/powerpc/pmu/ebb/reg_access_test'.
  make[2]: *** No rule to make target 'busy_loop.S', needed by '/home/linux-master/tools/testing/selftests/powerpc/pmu/ebb/reg_access_test'.
  make[2]: Target 'all' not remade because of errors.

Fix this by adding a suffix to the OUTPUT directory so that the
failure is avoided.

Fixes: 9686813f ("selftests/powerpc: Fix try-run when source tree is not writable")
Signed-off-by: Harish <harish@linux.ibm.com>
[mpe: Mention that commit that triggered the breakage]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20200625165721.264904-1-harish@linux.ibm.com