website: added instruction for building m5

Due migrating to scons in the gem5-20 release, the command to build
the m5 binary has changed. This commit adds the instructions on how
to build the m5 utility and what architectures are supported.

Jira: https://gem5.atlassian.net/browse/GEM5-523
Change-Id: I9f525511f6a237631aea577438cc30a6f3608ef1
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5-website/+/29212
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: Jason Lowe-Power <power.jg@gmail.com>
1 file changed