website: Update building doc to reflect .prof/.perf removal

As of this change:
https://gem5-review.googlesource.com/c/public/gem5/+/51988 the perf and
prof builds have been removed and replaced with `--gprof` and `--pprof`
compilation flags. This patch updates the building documentation to
reflect this.

Change-Id: Ia88b7b1769edac688dbaad4b899de4812599ec61
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5-website/+/55685
Reviewed-by: Gabe Black <gabe.black@gmail.com>
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: Jason Lowe-Power <power.jg@gmail.com>
Maintainer: Bobby Bruce <bbruce@ucdavis.edu>
1 file changed