RuntimeDyldELF: don't create thunk if not needed

This patch doesn't create thunk for branch operation when following conditions are met:
- Architecture is AArch64
- Relocation target is in the same object file
- Relocation target is close enough to be encoded in immediate offset

In such case we branch directly to the target instead of branching to thunk

Differential revision: https://reviews.llvm.org/D28108

llvm-svn: 291431