[RISCV] Support linker relax function call from auipc and jalr to jal

To do this:

1. Add fixup_riscv_relax fixup types which eventually will transfer to R_RISCV_RELAX relocation types.

2. 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