selftests/powerpc: Fix build with USERCFLAGS set

Currently building the powerpc selftests with USERCFLAGS set to anything
causes the build to break:

  $ make -C tools/testing/selftests/powerpc V=1 USERCFLAGS=-Wno-error
  ...
  gcc -Wno-error    cache_shape.c ...
  cache_shape.c:18:10: fatal error: utils.h: No such file or directory
     18 | #include "utils.h"
        |          ^~~~~~~~~
  compilation terminated.

This happens because the USERCFLAGS are added to CFLAGS in lib.mk, which
causes the check of CFLAGS in powerpc/flags.mk to skip setting CFLAGS at
all, resulting in none of the usual CFLAGS being passed. That can
be seen in the output above, the only flag passed to the compiler is
-Wno-error.

Fix it by dropping the conditional setting of CFLAGS in flags.mk.
Instead always set CFLAGS, but also append USERCFLAGS if they are set.

Note that appending to CFLAGS (with +=) wouldn't work, because flags.mk
is included by multiple Makefiles (to support partial builds), causing
CFLAGS to be appended to multiple times. Additionally that would place
the USERCFLAGS prior to the standard CFLAGS, meaning the USERCFLAGS
couldn't override the standard flags. Being able to override the
standard flags is desirable, for example for adding -Wno-error.

With the fix in place, the CFLAGS are set correctly, including the
USERCFLAGS:

  $ make -C tools/testing/selftests/powerpc V=1 USERCFLAGS=-Wno-error
  ...
  gcc -std=gnu99 -O2 -Wall -Werror -DGIT_VERSION='"v6.10-rc2-7-gdea17e7e56c3"'
  -I/home/michael/linux/tools/testing/selftests/powerpc/include -Wno-error
  cache_shape.c ...

Fixes: 5553a79387e9 ("selftests/powerpc: Add flags.mk to support pmu buildable")
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20240706120833.909853-1-mpe@ellerman.id.au

diff --git a/tools/testing/selftests/powerpc/flags.mk b/tools/testing/selftests/powerpc/flags.mk
index b909bee..abb9e58 100644
--- a/tools/testing/selftests/powerpc/flags.mk
+++ b/tools/testing/selftests/powerpc/flags.mk
@@ -5,8 +5,5 @@
 export GIT_VERSION
 endif
 
-ifeq ($(CFLAGS),)
-CFLAGS := -std=gnu99 -O2 -Wall -Werror -DGIT_VERSION='"$(GIT_VERSION)"' -I$(selfdir)/powerpc/include $(CFLAGS)
+CFLAGS := -std=gnu99 -O2 -Wall -Werror -DGIT_VERSION='"$(GIT_VERSION)"' -I$(selfdir)/powerpc/include $(USERCFLAGS)
 export CFLAGS
-endif
-