firefox: add profile-guided optimization

Lo and behold, we're finally catching up with Mozillas very own firefox
build in terms of speed.

PGO is an optimization technique in which in a first step we create a
build that supports instrumentation, meaning we can use it to create a
profile of how the browser behaved during usage. Then in a second pass
we create the final build that uses the acquired profiling data to
optimize the browser for the workload it actually received during
profiling.

The downside is that with PGO we now need to build Firefox twice, which
increases the build time from around 20 minutes to roughly 50 minutes.

In the Speedometer 2.0 benchmark multiple tests could see a
responsiveness improvemeant around 20-25%, which makes the increased
build time well worth it.

Sadly this benefit seems limited to x86_64-linux, builds on
aarch64-linux get stuck during profiling and I haven't found out why.

Finally, after a long time, we can say:

Closes: #76484
Supersedes: #129503

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