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>

include/linux/syscalls.h

@@ -696,10 +696,7 @@ asmlinkage long sys_pipe(int __user *);
 int kernel_execve(const char *filename, char *const argv[], char *const envp[]);
 
 
-asmlinkage int sys_perf_counter_open(
-
-	struct perf_counter_hw_event	*hw_event_uptr		__user,
-	pid_t				pid,
-	int				cpu,
-	int				group_fd);
+asmlinkage long sys_perf_counter_open(
+		const struct perf_counter_hw_event __user *hw_event_uptr,
+		pid_t pid, int cpu, int group_fd);
 #endif