aotcompile: remove most offset code (#52865)
Use absolute addresses for fvars, to avoid needing some of the copies
and complexity around them, and the buggy-ness of LLVM linker features
with doing this.
This may replace
https://github.com/JuliaLang/llvm-project/commit/a5ccdfbba7e54ea4792ff4bd61e163d209af36e1
