RISC-V: Reorder the prefixed extensions which are out of order.

RISC-V: Reorder the prefixed extensions which are out of order.

This patch has been pending for almost a year...  However, I noticed that
llvm can already re-order the extensions, even if they are out of orders.
Not really sure if they can also re-order the single letter extensions,
but at least we can do this for the multi-letter extensions in binutils.

bfd/
    * elfxx-riscv.c (riscv_parse_prefixed_ext): Removed the code which are
    used to check the prefixed extension orders.
gas/
    * testsuite/gas/riscv/march-fail-order-x-z.d: Removed since we will help
    tp reorder the prefixed extensions for now.
    * testsuite/gas/riscv/march-fail-order-x-z.l: Likewise.
    * testsuite/gas/riscv/march-fail-order-x.d: Likewise.
    * testsuite/gas/riscv/march-fail-order-x.l: Likewise.
    * testsuite/gas/riscv/march-fail-order-z.d: Likewise.
    * testsuite/gas/riscv/march-fail-order-z.l: Likewise.