tests: Add test program for C++ threads

Simple program that spawns threads equal to the number of CPU cores and
has some false sharing for testing coherence protocols.

Change-Id: I5be907fd6fea9a8b8e80b63785d186619be41354
Signed-off-by: Jason Lowe-Power <jason@lowepower.com>
Reviewed-on: https://gem5-review.googlesource.com/8901
Reviewed-by: Daniel Carvalho <odanrc@yahoo.com.br>