The upgrade to clang 14 showed that fdlibm is not necessarily free of fingerprinting problems depending how it's compiled. This needs to be carefully reviewed.

Until clang 14, the default was -ffp-contract=fast, but some reason
didn't really enable FP contraction in practice. GCC has the same
default, but does, in fact enable FP contraction*. This leads to more
inconsistencies in FP results between builds than would normally be
expected, and has an effect on e.g. fingerprinting resistance in fdlibm
trigonometric functions, or some other places (e.g. bug 1765436).

As far as the Firefox versions Mozilla ships are concerned, FP
contraction only really affects aarch64, but this change should in
practice make no difference with the current version of clang used (13),
and would preserve our now new default behavior with clang 14.

(*) We never noticed because we neither build with GCC for arm64 nor run
tests with GCC builds.

https://hg.mozilla.org/mozilla-central/rev/4b684fca9289