util: Implement Lua module for m5ops.

This module allows m5ops to be executed in Lua programs.
To compile it (in util/m5):
The following command generates Lua moduel, gem5OpLua.so.

make -f Makefile.<arch> gem5OpLua.so

To use it:
First, put gem5OpLua.so in Lua library search path.
Then, import the module and execute the m5op function.

Example usage, creating a checkpoint.

m5 = require("gem5OpLua")
m5.do_checkpoint(0, 0)

Change-Id: Icc18a1fb6c050afeb1cf4558fbdc724fb26a90e2
Reviewed-on: https://gem5-review.googlesource.com/6541
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>