sim: Add generic EventFunctionWrapper

sim: Add generic EventFunctionWrapper

Add EventFunctionWrapper, an event wrapper which takes any callable
object to use as its callback. (This includes c++ lambdas, function
pointers, bound functions, and std::functions.)

Change-Id: Iab140df47bd0f7e4b3fe3b568f9dd122a43cee1c
Signed-off-by: Sean Wilson <spwilson2@wisc.edu>
Reviewed-on: https://gem5-review.googlesource.com/3743
Reviewed-by: Anthony Gutierrez <anthony.gutierrez@amd.com>
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Maintainer: Anthony Gutierrez <anthony.gutierrez@amd.com>