Swap the r10 and rcx registers in the amd64 trapframe so that the

Swap the r10 and rcx registers in the amd64 trapframe so that the
first six entries are in the same order as syscall arguments, such
that syscall() can just use the trapframe as the argument vector
for mi_syscall() and not need to reorder into another buffer on the
stack.  This doesn't affect coredump layout or ptrace(2), but does
affect kernel crash dumps.

Possibility noted during miod@'s cleanup of the MD syscall()
implementations

ok mlarkin@ kurt@