aotcompile: remove most offset code

[vtjnash]

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.

@vchuravy this may replace JuliaLang/llvm-project@a5ccdfb

[vtjnash]

The old code was trying to approximate the normal relocations for this (e.g. the @__ImageBase = external dso_local constant i8 symbol at https://github.com/JuliaLang/llvm-project/blob/85122d41d6783e828412f9a1d0f7c771e2a38acc/clang/lib/CodeGen/MicrosoftCXXABI.cpp#L579-L594) but had somewhat bit-rotted in design, and wasn't really saving anything over doing the simple thing and keeping them as pointers