x86-64: Fix get_stack_args_count()

Signed-off-by: Pekka Enberg <penberg@kernel.org>
x86-64: Fix stack alignment for stack arguments

GCC assumes the stack is aligned to 16-bytes:

  http://gcc.gnu.org/bugzilla/show_bug.cgi?id=40838

Signed-off-by: Pekka Enberg <penberg@kernel.org>