Make truss work for CloudABI executables on i386.

The system call convention is different from i386 binaries running on
FreeBSD/amd64, but this is not noticeable by executables. On
FreeBSD/amd64, the vDSO already does padding of arguments and return
values to 64-bit values. On i386, it does not, meaning that system call
return values are simply stored in registers.