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.

include/elf/riscv.h

@@ -121,6 +121,9 @@ END_RELOC_NUMBERS (R_RISCV_max)
 /* RISC-V specific values for st_other.  */
 #define STO_RISCV_VARIANT_CC 0x80
 
+/* File uses the TSO model. */
+#define EF_RISCV_TSO 0x0010
+
 /* Additional section types.  */
 #define SHT_RISCV_ATTRIBUTES 0x70000003 /* Section holds attributes.  */