- Add detection of -fprofile-arcs/-fprofile-generate to configure

- Add a new client.mk target, profiledbuild, which does a profiling build, runs a specified script to generate the profile, and then rebuilds using the profile data.
- Add a profiling script for Seamonkey/Firefox that runs a couple cycles of the pageload test, and then does a second start to hit the fastload/history code.

Bug 237239, r=dbaron, sr=brendan, a=brendan.