cmd/compile/internal-abi: fix ABI0-equivalence for zero-sized values

This fixes a bug in the internal ABI specification that made it not
equivalent to ABI0 even with zero architectural argument registers in
the case of a zero-sized argument with alignment > 1.

In ABI0, even zero-sized arguments cause alignment padding in the
stack frame.

Currently, in the internal ABI, zero-sized arguments get
register-assigned even if there are no registers because they don't
consume any registers. Hence, they don't create alignment padding in
the stack frame.

Fix this by stack-assigning zero-sized arguments.

For #40724.

Change-Id: I1f5a95a94fed8b5313a360e5e76875701ba5f562
Reviewed-on: https://go-review.googlesource.com/c/go/+/295791
Trust: Austin Clements <austin@google.com>
Reviewed-by: Cherry Zhang <cherryyz@google.com>
Reviewed-by: Than McIntosh <thanm@google.com>

src/cmd/compile/internal-abi.md

@@ -153,6 +153,7 @@ Assigning a receiver, argument, or result V of underlying type T works
 as follows:
 
 1. Remember I and FP.
+1. If T has zero size, add T to the stack sequence S and return.
 1. Try to register-assign V.
 1. If step 2 failed, reset I and FP to the values from step 1, add T
    to the stack sequence S, and assign V to this field in S.
@@ -295,6 +296,15 @@ An architecture may still define register meanings that aren’t
 compatible with ABI0, but these differences should be easy to account
 for in the compiler.
 
+The assignment algorithm assigns zero-sized values to the stack
+(assignment step 2) in order to support ABI0-equivalence.
+While these values take no space themselves, they do result in
+alignment padding on the stack in ABI0.
+Without this step, the internal ABI would register-assign zero-sized
+values even on architectures that provide no argument registers
+because they don't consume any registers, and hence not add alignment
+padding to the stack.
+
 The algorithm reserves spill space for arguments in the caller’s frame
 so that the compiler can generate a stack growth path that spills into
 this reserved space.