RISC-V: Implement Ztso extension

RISC-V: Implement Ztso extension

This patch support ZTSO extension. It will turn on the tso flag for elf_flags
once we have enabled Ztso extension.  This is intended to implement v0.1 of
the proposed specification which can be found in Chapter 25 of,
https://github.com/riscv/riscv-isa-manual/releases/download/draft-20220723-10eea63/riscv-spec.pdf.

bfd\ChangeLog:

        * elfnn-riscv.c (_bfd_riscv_elf_merge_private_bfd_data): Set TSO flag.
        * elfxx-riscv.c: Add Ztso's arch.

binutils\ChangeLog:

        * readelf.c (get_machine_flags): Set TSO flag.

gas\ChangeLog:

        * config/tc-riscv.c (riscv_set_tso): Ditto.
        (riscv_set_arch): Ditto.
        * testsuite/gas/riscv/ztso.d: New test.

include\ChangeLog:

        * elf/riscv.h (EF_RISCV_TSO): Ditto.