RISC-V: Maintain a string to hold the canonical order

Using dummy entry in riscv_supported_std_ext cause confusing and wrongly
support `b` and `k` extensions.

bfd/
	* elfxx-riscv.c (riscv_supported_std_ext): Drop unsupported
	extensions.
	(riscv_ext_canonical_order): New.
	(riscv_init_ext_order): Use riscv_ext_canonical_order rather
	than riscv_supported_std_ext to compute canonical order.

V2 Changes:

- Use `*ext` rather than `*ext != NULL` for checking is reach end of
  string.