selftests/powerpc: Add flags.mk to support pmu buildable

When running `make -C powerpc/pmu run_tests` from top level selftests
directory, currently this error is being reported:

  make: Entering directory '/home/maddy/linux/tools/testing/selftests/powerpc/pmu'
  Makefile:40: warning: overriding recipe for target 'emit_tests'
  ../../lib.mk:111: warning: ignoring old recipe for target 'emit_tests'
  gcc -m64    count_instructions.c ../harness.c event.c lib.c ../utils.c loop.S  -o /home/maddy/selftest_output//count_instructions
  In file included from count_instructions.c:13:
  event.h:12:10: fatal error: utils.h: No such file or directory
  12 | #include "utils.h"
    |          ^~~~~~~~~
  compilation terminated.

This is due to missing of include path in CFLAGS. That is, CFLAGS and
GIT_VERSION macros are defined in the powerpc/ folder Makefile which
in this case is not involved.

To address the failure in case of executing specific sub-folder test
directly, a new rule file has been addded by the patch called "flags.mk"
under selftest/powerpc/ folder and is linked to all the Makefile of
powerpc/pmu sub-folders.

Reported-by: Sachin Sant <sachinp@linux.ibm.com>
Signed-off-by: Madhavan Srinivasan <maddy@linux.ibm.com>
Tested-by: Sachin Sant <sachinp@linux.ibm.com>
[mpe: Fixup ifeq, make GIT_VERSION simply expanded to avoid re-executing git describe]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20240229093711.581230-2-maddy@linux.ibm.com