[JITLink][RISCV] Use hashmap to find PCREL_HI20 edge (#78849)

As noted in issues #68594 and #73935, `JITLink/RISCV/ELF_ehframe.s`
fails with libstdc++'s expensive checks because `getRISCVPCRelHi20`
calls `std::equal_range` on the edges which may not be ordered by their
offset. Instead let `ELFJITLinker_riscv` build a hashmap of all edges
with type `R_RISCV_PCREL_HI20` that can be looked up in constant time.

Closes #73935