RISC-V: Set EF_RISCV_TSO also on .option arch

This is a minor fix to commit 96462b0
("RISC-V: Implement Ztso extension").  Currently, it sets EF_RISCV_TSO ELF
flag when initial ISA string contains the 'Ztso' extension.  However, GAS
has a way to update the ISA string: ".option arch".

When the architecture is updated by ".option arch", EF_RISCV_RVC ELF flag
is set when the 'C' extension is detected.  Analogously, this commit sets
the EF_RISCV_TSO when the 'Ztso' extension is detected.

gas/ChangeLog:

	* config/tc-riscv.c (s_riscv_option): Set TSO ELF flag if the
	'Ztso' extension is specified via ".option arch" directive.

gas/config/tc-riscv.c

@@ -3888,6 +3888,9 @@ s_riscv_option (int x ATTRIBUTE_UNUSED)
       riscv_set_rvc (false);
       if (riscv_subset_supports (&riscv_rps_as, "c"))
 	riscv_set_rvc (true);
+
+      if (riscv_subset_supports (&riscv_rps_as, "ztso"))
+	riscv_set_tso ();
     }
   else if (strcmp (name, "push") == 0)
     {