powerpc: syscall support up to 6 arguments

As reported by Kristof Provost.

Signed-off-by: Bernhard Reutner-Fischer <rep.dot.nop@gmail.com>

libc/sysdeps/linux/powerpc/syscall.S

@@ -29,6 +29,7 @@ syscall:
 	mr 5,6
 	mr 6,7
 	mr 7,8
+	mr 8,9
 	sc
 	bnslr;