RISC-V: Set EF_RISCV_TSO also on .option arch

RISC-V: Set EF_RISCV_TSO also on .option arch

This is a minor fix to commit 96462b012988d35ebb1137a2ad9fd0a96547d79a
("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.