perfcounters: fix a few minor cleanliness issues

This fixes three issues noticed by Arnd Bergmann:

- Add #ifdef __KERNEL__ and move some things around in perf_counter.h
  to make sure only the bits that userspace needs are exported to
  userspace.

- Use __u64, __s64, __u32 types in the structs exported to userspace
  rather than u64, s64, u32.

- Make the sys_perf_counter_open syscall available to the SPUs on
  Cell platforms.

And one issue that I noticed in looking at the code again:

- Wrap the perf_counter_open syscall with SYSCALL_DEFINE4 so we get
  the proper handling of int arguments on ppc64 (and some other 64-bit
  architectures).

Reported-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Paul Mackerras <paulus@samba.org>

kernel/perf_counter.c

@@ -1690,9 +1690,9 @@ perf_counter_alloc(struct perf_counter_hw_event *hw_event,
  * @cpu:		target cpu
  * @group_fd:		group leader counter fd
  */
-asmlinkage int
-sys_perf_counter_open(struct perf_counter_hw_event *hw_event_uptr __user,
-		      pid_t pid, int cpu, int group_fd)
+SYSCALL_DEFINE4(perf_counter_open,
+		const struct perf_counter_hw_event __user *, hw_event_uptr,
+		pid_t, pid, int, cpu, int, group_fd)
 {
 	struct perf_counter *counter, *group_leader;
 	struct perf_counter_hw_event hw_event;