Basically, a 1:1 copy of the Linux x86-64 one. The Apple docs say that
"The OS X x86-64 function calling conventions are the same as the
function calling conventions described in System V Application Binary
Interface AMD64 Architecture Processor Supplement," which should mean
that it's the same as on Linux.

Once this works, the very minor differences to the Linux one will be
merged as ifdefs into the Linux one, and no separate source files will
be needed.

Change-Id: I1ff58f303a7ab7f59ec2ff7a9e24465e89151ec5