Backport of D27629, commit 18805ea951.

Original commit message:
---
Makes sure that the unwind info uses 64bits pcrel relocation if a large
code model is specified and handle the corresponding relocation in the
ExecutionEngine. This can happen with certain kernel configuration (the
same as the one in https://reviews.llvm.org/D27609, found at least on
the ArchLinux stock kernel and the one used on https://www.packet.net/

)
using the builtin JIT memory manager.

Co-authored-by: Yichao Yu <yyc1992@gmail.com>
Co-authored-by: Valentin Churavy <v.churavy@gmail.com>
---

Note: The handling in ExecutionEngine was committed in a different
revision and is already part of LLVM 9. We need the part about emitting
relocations because eh_frame (allocated in a data section) may be more
than 4Gb away from the code section it references. See the discussion
in https://github.com/cms-sw/cmssw/issues/31123 for context.