perf bpf: Add bpf.h to be used in eBPF proggies

So, the first helper is the one shortening a variable/function section
attribute, from, for instance:

  char _license[] __attribute__((section("license"), used)) = "GPL";

to:

  char _license[] SEC("license") = "GPL";

Convert empty.c to that and it becomes:

  # cat ~acme/lib/examples/perf/bpf/empty.c
  #include <bpf.h>

  char _license[] SEC("license") = "GPL";
  int _version SEC("version") = LINUX_VERSION_CODE;
  #

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Wang Nan <wangnan0@huawei.com>
Link: https://lkml.kernel.org/n/tip-zmeg52dlvy51rdlhyumfl5yf@git.kernel.orgSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/Makefile.perf

@@ -770,6 +770,7 @@ endif
 ifndef NO_LIBBPF
 	$(call QUIET_INSTALL, lib) \
 		$(INSTALL) -d -m 755 '$(DESTDIR_SQ)$(perf_include_instdir_SQ)/bpf'
+		$(INSTALL) include/bpf/*.h '$(DESTDIR_SQ)$(perf_include_instdir_SQ)/bpf'
 	$(call QUIET_INSTALL, lib) \
 		$(INSTALL) -d -m 755 '$(DESTDIR_SQ)$(perf_examples_instdir_SQ)/bpf'
 		$(INSTALL) examples/bpf/*.c '$(DESTDIR_SQ)$(perf_examples_instdir_SQ)/bpf'

tools/perf/examples/bpf/empty.c

-char _license[] __attribute__((section("license"), used)) = "GPL";
-int _version __attribute__((section("version"), used)) = LINUX_VERSION_CODE;
+#include <bpf.h>
+
+char _license[] SEC("license") = "GPL";
+int _version SEC("version") = LINUX_VERSION_CODE;

tools/perf/include/bpf/bpf.h

+// SPDX-License-Identifier: GPL-2.0
+#ifndef _PERF_BPF_H
+#define _PERF_BPF_H
+#define SEC(NAME) __attribute__((section(NAME),  used))
+#endif /* _PERF_BPF_H */