[RTDYLD] support absolute relocations where needed
These appear in some sections, such as DWARF tables, since
RuntimeDyldELF explicitly maps to this as a sentinel value:
That could then be a source of problems if it tried to examine these
sections (for example, with either setProcessAllSections(true) or ORCv2 on i686).
Reviewed By: lhames, vchuravy
Differential Revision: https://reviews.llvm.org/D90722