cdecl calling convention expects the callee to pop the hidden pointer on

cdecl calling convention expects the callee to pop the hidden pointer on
struct return. For example, GCC and LLVM comply with this (tested on IA32).

ACK doesn't seem to follow this convention and expects the caller to clean up
the stack. Compiling hand-written ACK-compliant assembly code (returning a
struct) with GCC or LLVM used to break things (4-bytes misaligned stack).

The patch fixes this problem.