Deprecate split objects
For several reasons (see below) we now generally recommend users use split sections instead of split objects.
- SplitSections is quite a bit simpler
- SplitSections may produce smaller binaries
- SplitSections requires fewer assembler invocations, potentially improving compilation time (see #11285 (closed), #15051 (closed))
- SplitSections is the approach used by most C compilers, meaning we may avoid edge cases that SplitObjects may turn up