perf tools: Move barrier() definition to tools/include/linux/compiler.h

perf tools: Move barrier() definition to tools/include/linux/compiler.h

To make it generally accessible by other tools/ projects, also will be
used in the tools/arch/*/include/asm/barrier.h files that are being
introduced now.

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Borislav Petkov <bp@suse.de>
Cc: David Ahern <dsahern@gmail.com>
Cc: Don Zickus <dzickus@redhat.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Stephane Eranian <eranian@google.com>
Link: http://lkml.kernel.org/n/tip-qnjdqwu3vcnt14vqmr6wu788@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>