Bug 923080 - Generate xpt files into final location. r=glandium

Now that the mozbuild backend knows about FINAL_TARGET, we are able to
install generated xpt files into their final location. This saves us
from copying xpt files into their final location on every build.

Original patch by gps, rebased and comments addressed by Ms2ger

--HG--
extra : transplant_source : %E2%DC%0F%E0%AD%C2%25%A1%B8%A9%FE%B0%8C%60%FF%CB%02G%25%E5
Bug 1042696 - Set allocator to TextureClient from constructor. r=nical
Bug 1113728 - Add pagespeedmobilizer.com to PSL. r=gerv
Bug 1113200 - Use sysconf to get the page size. r=dhylands
Bug 1111607 - Searchplugins should be picked up from en-US, if possible. r=glandium

--HG--
extra : transplant_source : %C9%23%D72%E7%EF%88%C8%1F%CFa_%1E%9B%0Cf%22%A0%BFu