Bug 1507334 - Add mozconfigs for 3-stage PGO; r=firefox-build-system-reviewers,chmanchester#firefox-build-system-reviewers

These two mozconfigs are used for the 1st and 3rd stages of PGO.
'profile-generate' is used for the 1st stage to generate a firefox
binary with instrumentation enabled. Since it is only used for getting
profile data, we don't run tests against it or need crashreporter
symbol.s

The 'profile-use' mozconfig is for the 3rd and final stage were we
produce the fully optimized build. The profiling information and jarlog
are input artifacts produced by the 2nd stage (running the
profileserver).

Differential Revision: https://phabricator.services.mozilla.com/D15749

--HG--
extra : moz-landing-system : lando