[POWERPC] Change ppc_rtas declaration to weak

Change the definition of powerpc's cond_syscall() to use the standard gcc
weak attribute specifier which provides proper support for C linkage as
needed by spu_syscall_table[].

Fixes this powerpc build error with CONFIG_SPU_FS=y, CONFIG_PPC_RTAS=n:

 arch/powerpc/platforms/built-in.o: undefined reference to `ppc_rtas'
Signed-off-by: Geoff Levand <geoffrey.levand@am.sony.com>
Signed-off-by: Arnd Bergmann <arnd.bergmann@de.ibm.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>

include/asm-powerpc/unistd.h

@@ -446,7 +446,6 @@ type name(type1 arg1, type2 arg2, type3 arg3, type4 arg4, type5 arg5, type6 arg6
 #include <linux/types.h>
 #include <linux/compiler.h>
 #include <linux/linkage.h>
-#include <asm/syscalls.h>
 
 #define __ARCH_WANT_IPC_PARSE_VERSION
 #define __ARCH_WANT_OLD_READDIR
@@ -481,16 +480,9 @@ type name(type1 arg1, type2 arg2, type3 arg3, type4 arg4, type5 arg5, type6 arg6
 
 /*
  * "Conditional" syscalls
- *
- * What we want is __attribute__((weak,alias("sys_ni_syscall"))),
- * but it doesn't work on all toolchains, so we just do it by hand
  */
-#ifdef CONFIG_PPC32
-#define cond_syscall(x) asm(".weak\t" #x "\n\t.set\t" #x ",sys_ni_syscall")
-#else
-#define cond_syscall(x) asm(".weak\t." #x "\n\t.set\t." #x ",.sys_ni_syscall")
-#endif
-
+#define cond_syscall(x) \
+	asmlinkage long x (void) __attribute__((weak,alias("sys_ni_syscall")))
 
 #endif		/* __ASSEMBLY__ */
 #endif		/* __KERNEL__ */