It was possible for `arg6` to be passed as an operand relative to esp.
In that case, the `push` at the top clobbered esp and hence made the
reference to arg6 invalid. This was manifesting in this branch as broken
stack traces on x86-linux due to an `mmap2` syscall accidentally passing
the page offset as non-zero!

This commit fixes a bug introduced in cb0e6d8aa.