[NativeAOT-LLVM] `WasmObjectWriter` efficiency improvements

[SingleAccretion]

1. LinkingSectionChunks are always stored in arrays. This is wasteful for cases where there can only be one chunk (such as in the code section).
2. FunctionTypeIndex and LinkingSectionRelativeOffset can share the same underlying storage, since they're used for two distinct types of symbols.
3. The current two pass model runs each emission method twice. Would it be faster to reserve 5 byte LEBs for section sizes and then seek back to overwrite them instead?

I am leaving this up-for-grabs for now.