RISC-V: Fix the merged orders of Z* extension for linker.

Similar to the commit 6729e2c2,
we have to check the first char of the Z* extensions, to make
sure that they follow the order of the standard extensions.

bfd/
    * elfxx-riscv.c (riscv_compare_subsets): Removed static.
    * elfxx-riscv.h: Add declaration.
    * elfnn-riscv.c (riscv_merge_multi_letter_ext): Use
    riscv_compare_subsets to check the orders.
    (riscv_skip_prefix): Removed.
    (riscv_prefix_cmp): Removed.