perf tools: Let python use correct gcc for build_ext

Currently, python uses host gcc instead of cross-compile gcc in the last
step of compiling build_ext(remove '--quiet' to show verbose):

  cross-gcc ...
  cross-gcc ...
  creating ~/out/python_ext_build/lib
  gcc -pthread -shared -Wl,-z ...

This is wrong but may not cause any errors unless the features detected
by cross-compiler do not match those for host compiler, and causes the
following errors:

  /usr/lib64/gcc/bin/ld: cannot find -lunwind-x86
  collect2: error: ld returned 1 exit status
  error: command 'gcc' failed with exit status 1
  cp: cannot stat ‘~/out/python_ext_build/lib/perf.so’: No such file or directory
  Makefile.perf:257: recipe for target '~/out/python/perf.so' failed
  make[1]: *** [~/out/python/perf.so] Error 1
  Makefile:68: recipe for target 'all' failed
  make: *** [all] Error 2

This issue is also reported and anwsered on stackoverflow.
Link: http://stackoverflow.com/questions/5986256/python-distutils-gcc-pathSigned-off-by: He Kuang <hekuang@huawei.com>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Ekaterina Tumanova <tumanova@linux.vnet.ibm.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Kan Liang <kan.liang@intel.com>
Cc: Masami Hiramatsu <mhiramat@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Pekka Enberg <penberg@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Stephane Eranian <eranian@google.com>
Cc: Sukadev Bhattiprolu <sukadev@linux.vnet.ibm.com>
Cc: Wang Nan <wangnan0@huawei.com>
Link: http://lkml.kernel.org/r/1466578626-92406-5-git-send-email-hekuang@huawei.comSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/Makefile.perf

@@ -254,7 +254,8 @@ PYTHON_EXT_SRCS := $(shell grep -v ^\# util/python-ext-sources)
 PYTHON_EXT_DEPS := util/python-ext-sources util/setup.py $(LIBTRACEEVENT) $(LIBAPI)
 
 $(OUTPUT)python/perf.so: $(PYTHON_EXT_SRCS) $(PYTHON_EXT_DEPS) $(LIBTRACEEVENT_DYNAMIC_LIST)
-	$(QUIET_GEN)CFLAGS='$(CFLAGS)' LDFLAGS='$(LDFLAGS) $(LIBTRACEEVENT_DYNAMIC_LIST_LDFLAGS)' \
+	$(QUIET_GEN)LDSHARED="$(CC) -pthread -shared" \
+        CFLAGS='$(CFLAGS)' LDFLAGS='$(LDFLAGS) $(LIBTRACEEVENT_DYNAMIC_LIST_LDFLAGS)' \
 	  $(PYTHON_WORD) util/setup.py \
 	  --quiet build_ext; \
 	mkdir -p $(OUTPUT)python && \