Build Tor Browser with PGO enabled on Linux

Mozilla is publishing the PGO profiles for Linux at least (Windows is supposed to follow with Firefox 69 and macOS has no PGO) and the build as of Firefox 68 is still reproducible (including LTO): https://glandium.org/blog/?p=3923.

We should try this out for performance improvements.

To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information