RISC-V: Record implicit subsets in a table, to avoid repeated codes.

Add a new table, riscv_implicit_subsets, to record all implicit information.
So that we add all implicit subsets according to the table, to avoid too
many repeated codes in the riscv_parse_add_implicit_subsets.  Besides, the
check_func is used to check whether we should add this implicit subset.
For example, check_implicit_for_i checks the version of i, and we only add
zicsr and zifencei implicitly only when the version less than 2.1.

bfd/
    * elfxx-riscv.c (check_implicit_always): The check_func, always add
    the implicit subset without checking.
    (check_implicit_for_i): The check_func for i, only add zicsr and
    zifencei when the version of i less than 2.1.
    (struct riscv_implicit_subset): Record the subsets and their
    corresponding implicit subsets.
    (riscv_implicit_subsets): Table records all implicit informations.
    (riscv_parse_add_implicit_subsets): Updated and add implicit subsets
    according to riscv_implicit_subsets.  Remove the redundant codes.