selftests/powerpc: make sub-folders buildable on their own

selftests/powerpc: make sub-folders buildable on their own

Build breaks when executing make with run_tests for sub-folders
under powerpc. This is because, CFLAGS and GIT_VERSION macros are
defined in Makefile of toplevel powerpc folder.

  make: Entering directory '/home/maddy/linux/tools/testing/selftests/powerpc/mm'
  gcc     hugetlb_vs_thp_test.c ../harness.c ../utils.c  -o /home/maddy/selftest_output//hugetlb_vs_thp_test
  hugetlb_vs_thp_test.c:6:10: fatal error: utils.h: No such file or directory
      6 | #include "utils.h"
        |          ^~~~~~~~~
  compilation terminated.

Fix this by adding the flags.mk in each sub-folder Makefile. Also remove
the CFLAGS and GIT_VERSION macros from powerpc/ folder Makefile since
the same is definied in flags.mk

Signed-off-by: Madhavan Srinivasan <maddy@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20240229093711.581230-3-maddy@linux.ibm.com