tools build: Check if gettid() is available before providing helper

Laura reported that the perf build failed in fedora when we got a glibc
that provides gettid(), which I reproduced using fedora rawhide with the
glibc-devel-2.29.9000-26.fc31.x86_64 package.

Add a feature check to avoid providing a gettid() helper in such
systems.

On a fedora rawhide system with this patch applied we now get:

[root@7a5f55352234 perf]# grep gettid /tmp/build/perf/FEATURE-DUMP
feature-gettid=1
[root@7a5f55352234 perf]# cat /tmp/build/perf/feature/test-gettid.make.output
[root@7a5f55352234 perf]# ldd /tmp/build/perf/feature/test-gettid.bin
linux-vdso.so.1 (0x00007ffc6b1f6000)
libc.so.6 => /lib64/libc.so.6 (0x00007f04e0a74000)
/lib64/ld-linux-x86-64.so.2 (0x00007f04e0c47000)
[root@7a5f55352234 perf]# nm /tmp/build/perf/feature/test-gettid.bin | grep -w gettid
U gettid@@GLIBC_2.30
[root@7a5f55352234 perf]#

While on a fedora:29 system:

[acme@quaco perf]$ grep gettid /tmp/build/perf/FEATURE-DUMP
feature-gettid=0
[acme@quaco perf]$ cat /tmp/build/perf/feature/test-gettid.make.output
test-gettid.c: In function ‘main’:
test-gettid.c:8:9: error: implicit declaration of function ‘gettid’; did you mean ‘getgid’? [-Werror=implicit-function-declaration]
return gettid();
^~~~~~
getgid
cc1: all warnings being treated as errors
[acme@quaco perf]$

Reported-by: Laura Abbott <[e-mail]>
Tested-by: Laura Abbott <[e-mail]>
Acked-by: Jiri Olsa <[e-mail]>
Cc: Adrian Hunter <[e-mail]>
Cc: Florian Weimer <[e-mail]>
Cc: Namhyung Kim <[e-mail]>
Cc: Stephane Eranian <[e-mail]>
Link: https://lkml.kernel.org/n/[e-mail]
Signed-off-by: Arnaldo Carvalho de Melo <[e-mail]>

tools/build/Makefile.feature

···
         fortify-source                  \
         sync-compare-and-swap           \
         get_current_dir_name            \
+        gettid				\
         glibc                           \
         gtk2                            \
         gtk2-infobar                    \

tools/build/feature/Makefile

···
          test-get_cpuid.bin                     \
          test-sdt.bin                           \
          test-cxx.bin                           \
+         test-gettid.bin			\
          test-jvmti.bin				\
          test-jvmti-cmlr.bin			\
          test-sched_getcpu.bin			\
···
 
 $(OUTPUT)test-cxx.bin:
 	$(BUILDXX) -std=gnu++11
+
+$(OUTPUT)test-gettid.bin:
+	$(BUILD)
 
 $(OUTPUT)test-jvmti.bin:
 	$(BUILD)

tools/build/feature/test-all.c

···
 # include "test-get_current_dir_name.c"
 #undef main
 
+#define main main_test_gettid
+# include "test-gettid.c"
+#undef main
+
 #define main main_test_glibc
 # include "test-glibc.c"
 #undef main
···
 	main_test_libelf();
 	main_test_libelf_mmap();
 	main_test_get_current_dir_name();
+	main_test_gettid();
 	main_test_glibc();
 	main_test_dwarf();
 	main_test_dwarf_getlocations();

tools/build/feature/test-gettid.c

···
+// SPDX-License-Identifier: GPL-2.0
+// Copyright (C) 2019, Red Hat Inc, Arnaldo Carvalho de Melo <[e-mail]>
+#define _GNU_SOURCE
+#include <unistd.h>
+
+int main(void)
+{
+	return gettid();
+}
+
+#undef _GNU_SOURCE

tools/perf/Makefile.config

···
   CFLAGS += -DHAVE_GET_CURRENT_DIR_NAME
 endif
 
+ifeq ($(feature-gettid), 1)
+  CFLAGS += -DHAVE_GETTID
+endif
+
 ifdef NO_LIBELF
   NO_DWARF := 1
   NO_DEMANGLE := 1

tools/perf/jvmti/jvmti_agent.c

···
 static char jit_path[PATH_MAX];
 static void *marker_addr;
 
+#ifndef HAVE_GETTID
 static inline pid_t gettid(void)
 {
 	return (pid_t)syscall(__NR_gettid);
 }
+#endif
 
 static int get_e_machine(struct jitheader *hdr)
 {