x86_64 c2i/i2c adapters may use more stack space than necessary

While looking at another bug, I wondered why the c2i adapter for a static call with no arguments is pushing 16 bytes onto the stack:

   pop %rax
   mov %rsp,%r13
   sub $0x10,%rsp
   mov %rax,(%rsp)

This could be just:

   mov %rsp,%r13
   add $0x8, %r13

If I'm not mistaken, the alignment invariant is that stack is aligned before the return value is pushed, so the slot for the return value should not be included in the alignment adjustment.