RISC-V: Extract the ld code which are too complicated, and may be reused.

These types of codes are different for each target, I am not sure what are the
best for RISC-V, so extract them out may be more easy to compare what's the
difference.

bfd/
    * elfnn-riscv.c (RISCV_NEED_DYNAMIC_RELOC): New defined.  Extracted
    from riscv_elf_check_relocs, to see if dynamic reloc is needed for the
    specific relocation.
    (RISCV_GENERATE_DYNAMIC_RELOC): New defined.  Extracted from
    riscv_elf_relocate_section, to see if R_RISCV_32/64 need to generate
    dynamic relocation.
    (RISCV_COPY_INPUT_RELOC): New defined.  Extracted from
    riscv_elf_relocate_section, to see if R_RISCV_32/64 need to copy itslef
    tp output file.
    (RISCV_RESOLVED_LOCALLY): New defined.  Extracted from
    riscv_elf_relocate_section, to see if R_RISCV_GOT_HI20 can be resolved
    locally.