The value of an absolute relocation, like R_RISCV_HI20 or R_PPC64_LO16,
with a symbol index of 0, the resulting value should be treated as
absolute and permitted in both -pie and -shared links.
This change also resolves an absolute relocation referencing an
undefined symbol in statically-linked executables.
PPC64 has unfortunate exceptions:
* R_PPC64_TOCBASE uses symbol index 0 but it should be treated as
referencing the linker-defined .TOC.
* R_PPC64_PCREL_OPT (https://reviews.llvm.org/D84360) could no longer
rely on `isAbsoluteValue` return false.
ba2de8f22d