[Object][RISCV] Resolve R_RISCV_32_PCREL

Summary: 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.

Reviewers: asb, lenary
Reviewed By: asb
Tags: #llvm
Differential Revision: https://reviews.llvm.org/D70204