[llvm] [M68k] add 32 bit branch instrs and relaxations (PR #117371)

[via llvm-commits]

knickish wrote:

Copied most of the relocation tests to run them with `--position-independent`, and added some encoding tests. Also included some small tweaks to behavior that I found necessary during manual testing using the rust core library with `-fno-function-sections` related to branch relaxation/fixups and explicit casts for unsigned -> signed conversions. The original tests still pass, so no changes to existing functionality. 

The encoding tests themselves aren't that helpful as the assembly is emitted before fixup relaxation happens for the branches. Might be good to change the branch instructions to pattern match based on size at some point instead of always matching the 8bit version and relaxing

https://github.com/llvm/llvm-project/pull/117371