While the changes in rL347429 brought dramatic improvements to Firefox link time on ARM64, they also caused large regressions in link time on x86_64 PGO builds, severe enough that we had to back out our clang roll. I assume the slowdown has something to do with the sections created for profiling instrumentation. (It would be interesting to see what effect that patch had on ARM64 PGO builds, but we don't have such a thing yet.)
As a starting point, here's a patch to restrict rL347429 to ARM64 only. (And, if accepted, I'd suggest merging it to to 8.0.0.) Being unfamiliar with this code, I can't tell if this is a proper solution or a hack, so I'll rely on reviewers to let me know.