[RISCV] Make RISCVELFObjectWriter::getRelocType check IsPCRel

Previously, this function didn't check the IsPCRel argument. But doing so is a
useful check for errors, and also seemingly necessary for FK_Data_4 (which we
produce a R_RISCV_32_PCREL relocation for if IsPCRel).

Other than R_RISCV_32_PCREL, this should be NFC. Future exception handling
related patches will include tests that capture this behaviour.

llvm-svn: 366172