RISC-V: Refactor PLT generation

The goal of this refactor is to improve the possiblity of having
different PLT generation code for different RISC-V ABIs. The changes
include:
- Extract PLT generation logic into individual functions.
- Keep the PLT generation data in riscv_elf_link_hash_table.

In the following patches, we will use this framework to implement
different PLT.