This adapts the provided runtime.s to work with IA-32 macho calling conventions.
Description of syscalls can be found here: http://www.opensource.apple.com/source/xnu/xnu-1504.3.12/bsd/kern/syscalls.master
And calling convention can be found here: https://developer.apple.com/library/mac/documentation/DeveloperTools/Conceptual/LowLevelABI/130-IA-32_Function_Calling_Conventions/IA32.html
This seems to work, but for reasons I don't understand, aligning the stack to an 8 byte boundary seems to work, while aligning to the prescribed 16 byte boundary doesn't.