[ELF] Relax R_RISCV_CALL and R_RISCV_CALL_PLT

A pair of auipc+jalr relocated by R_RISCV_CALL or R_RISCV_CALL_PLT can be
converted to c.j, c.jal, or jal.

* c.j: RVC and displacement is representable as an int12
* c.jal: RV32C and displacement is representable as an int12
* jal: displacement is representable as an int21

Use the D127581 relaxation framework to implement the relaxation. If a shorter
sequence is satisfied, we record the new relocation type in `relocTypes` and
saves the new instruction into `writes`. Finally let `riscvFinalizeRelax` rewrite the
instruction by setting `skip`.

Differential Revision: https://reviews.llvm.org/D127611