In rust-lang#91608 the fp-armv8 feature was removed as it's tied to the neon
feature. However disabling neon didn't actually disable the use of
floating point registers and instructions, for this `-fp-armv8` is
required.

Some features that are tied together only make sense to be folded
together when enabling the feature. For example on AArch64 sve and
neon are tied together, however it doesn't make sense to disable neon
when disabling sve.

Rather than returning an array of features from to_llvm_features, return a structure that contains
the dependencies. This also contains metadata on how the features depend on each other to allow for
the correct enabling and disabling.