cpu: Enable fast-forwarding for MIPS InOrderCPU and O3CPU

A copyRegs() function is added to MIPS utilities
to copy architectural state from the old CPU to
the new CPU during fast-forwarding. This
addition alone enables fast-forwarding for the
o3 cpu model running MIPS.

The patch also adds takeOverFrom() and
drainResume() functions to the InOrderCPU to
enable it to take over from another CPU. This
change enables fast-forwarding for the inorder
cpu model running MIPS, but not for Alpha.

All quick and long regressions still pass for
all ISAs.

Using HelloWorld, I scaled the number of
instructions I fast-forward and see the
number of committed instructions scale
accordingly. The host_seconds stat also scales
accordingly, getting smaller as I fast-forward
for more instructions. This works for both the
MIPS InOrderCPU and O3CPU.