freebsd: Fix argc resolution in _start

FreeBSD appears to use rdi instead of rsp as in other posix systems.
According to some loose documentation, x86 passes values on the stack,
so amd64 freebsd may be the only exception.
19659c3bd3Marc Tiehuis committed on 10/20/2018, 12:15:01 PM· committed by Greg Vparent102cb61
2 files changedLine totals unavailable