Add support for resolving R_RISCV_32_PCREL relocations. Those aren't actually resolved AFAIK, but support is still needed to avoid llvm-dwarfdump errors. The use of these relocations was introduced in D66419 but the corresponding resolving wasn't added then*. The test adds a check that should catch future unresolved relocations.
*I would blame the reviewer, but it turns out that person was me, so he's exculpated.