perf s390: Fix TEXTREL in Python extension by compiling as PIC

On s390 the Python extension build fails as follows when using a linker
that is configured to treat text relocations (TEXTREL) in shared
libraries as error by default:

  GEN     python/perf.cpython-314-s390x-linux-gnu.so
/usr/bin/ld.bfd: error: read-only segment has dynamic relocations

This occurrs because util/llvm-c-helpers.o is erroneously built from
util/llvm-c-helpers.cpp without compiler option -fPIC but linked into
the shared library (via libperf-util.a(perf-util-in.o)).

On s390, object files must be compiled as position-indepedent code (PIC)
in order to be linked into shared libraries.  Commit a9a3f1d18a ("perf
s390: Always build with -fPIC") added compiler option -fPIC to CFLAGS
for s390, which is used in C compiles.  Add -fPIC to CXXFLAGS for s390
as well, so that it is also used in C++ compiles.

Fixes: a9a3f1d18a ("perf s390: Always build with -fPIC")
Reported-by: Thomas Richter <tmricht@linux.ibm.com>
Reviewed-by: Ian Rogers <irogers@google.com>
Reviewed-by: James Clark <james.clark@linaro.org>
Signed-off-by: Jens Remus <jremus@linux.ibm.com>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Bill Wendling <morbo@google.com>
Cc: Heiko Carstens <hca@linux.ibm.com>
Cc: Hendrik Brueckner <brueckner@linux.ibm.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Jan Polensky <japo@linux.ibm.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Justin Stitt <justinstitt@google.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Nathan Chancellor <nathan@kernel.org>
Cc: Nick Desaulniers <nick.desaulniers+lkml@gmail.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/Makefile.config

@@ -77,6 +77,7 @@ endif
 
 ifeq ($(ARCH),s390)
   CFLAGS += -fPIC
+  CXXFLAGS += -fPIC
 endif
 
 # Unconditionally set up the libunwind feature build flags as a