powerpc: Fix compiler warning on some syscalls

GCC 5.0 emits an warning when using sizeof on array function parameters
and powerpc internal syscall macros add a check for such cases.  More
specifically, on powerpc64 and powerpc32 sysdep.h:

  if (__builtin_classify_type (__arg3) != 5 && sizeof (__arg3) > 8) \
          __illegally_sized_syscall_arg3 (); \

And for sysdeps/unix/sysv/linux/utimensat.c build GCC emits:

error: ‘sizeof’ on array function parameter ‘tsp’ will return size of
‘const struct timespec *’

This patch uses the address of first struct member instead of the struct
itself in syscall macro.