[lld-macho] Rename SubsectionMapping to SubsectionMap

We bikeshedded about it here: https://reviews.llvm.org/D98837#inline-931557

I initially suggested SubsectionMapping, but I thought the discussion
landed on doing `std::vector<SubsectionEntry>`. @alexshap went and did
both, but on hindsight I regret adding 3 more characters to an already
long name, and I think SubsectionEntry is descriptive enough...

This diff also renames `subsectionMap` to `subsecMap` for consistency
with other variable names in the codebase.