scons: rename some things from m5 to gem5
scons: rename some things from m5 to gem5

The default generated binary is now gem5.<type> instead of m5.<type>.
The latter does still work but gem5.<type> will be generated first and
then m5.<type> will be hard linked to it.

pombredanne/http-repo.gem5.org-gem5-,hoangt/tpzsimul.gem5,hoangt/tpzsimul.gem5,pombredanne/http-repo.gem5.org-gem5-,hoangt/tpzsimul.gem5,hoangt/tpzsimul.gem5,pombredanne/http-repo.gem5.org-gem5-,vovojh/gem5,pombredanne/http-repo.gem5.org-gem5-,vovojh/gem5,vovojh/gem5,hoangt/tpzsimul.gem5,hoangt/tpzsimul.gem5,pombredanne/http-repo.gem5.org-gem5-,vovojh/gem5,vovojh/gem5,vovojh/gem5,vovojh/gem5,pombredanne/http-repo.gem5.org-gem5-,pombredanne/http-repo.gem5.org-gem5-,hoangt/tpzsimul.gem5