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 <[e-mail]>
Signed-off-by: Madhavan Srinivasan <[e-mail]>
Tested-by: Sachin Sant <[e-mail]>
[mpe: Fixup ifeq, make GIT_VERSION simply expanded to avoid re-executing git describe]
Signed-off-by: Michael Ellerman <[e-mail]>
Link: https://msgid.link/[e-mail]

tools/testing/selftests/powerpc/flags.mk

···
+#This checks for any ENV variables and add those.
+
+ifeq ($(GIT_VERSION),)
+GIT_VERSION := $(shell git describe --always --long --dirty || echo "unknown")
+export GIT_VERSION
+endif
+
+ifeq ($(CFLAGS),)
+CFLAGS := -std=gnu99 -O2 -Wall -Werror -DGIT_VERSION='"$(GIT_VERSION)"' -I$(selfdir)/powerpc/include $(CFLAGS)
+export CFLAGS
+endif
+

tools/testing/selftests/powerpc/pmu/Makefile

···
 
 top_srcdir = ../../../../..
 include ../../lib.mk
+include ../flags.mk
 
 all: $(TEST_GEN_PROGS) ebb sampling_tests event_code_tests
 

tools/testing/selftests/powerpc/pmu/ebb/Makefile

···
 
 top_srcdir = ../../../../../..
 include ../../../lib.mk
+include ../../flags.mk
 
 # The EBB handler is 64-bit code and everything links against it
 CFLAGS += -m64

tools/testing/selftests/powerpc/pmu/event_code_tests/Makefile

···
 
 top_srcdir = ../../../../../..
 include ../../../lib.mk
+include ../../flags.mk
 
 CFLAGS += -m64
 

tools/testing/selftests/powerpc/pmu/sampling_tests/Makefile

···
 
 top_srcdir = ../../../../../..
 include ../../../lib.mk
+include ../../flags.mk
 
 CFLAGS += -m64