[RISCV] Support linker relax function call from auipc and jalr to jal
To do this:
- Add fixup_riscv_relax fixup types which eventually will transfer to R_RISCV_RELAX relocation types.
- Insert R_RISCV_RELAX relocation types to auipc function call expression when linker relaxation enabled.
Differential Revision: https://reviews.llvm.org/D44886
llvm-svn: 333158