x86-64: Fix stack frame alignment for input arguments

Section 3.2.2 ("The Stack Frame") of the SVR4 x86-64 ABI states that:

  The end of the input argument area shall be aligned on a 16 byte
  boundary. In other words, the value (%rsp - 8) is always a multiple of
  16 when control is transferred to the function entry point. The stack
  pointer, %rsp, always points to the end of the latest allocated stack
  frame.

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