x86/srso: Add SRSO mitigation for Hygon processors

Add mitigation for the speculative return stack overflow vulnerability
which exists on Hygon processors too.

Signed-off-by: Pu Wen <puwen@hygon.cn>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Acked-by: Borislav Petkov (AMD) <bp@alien8.de>
Cc: <stable@vger.kernel.org>
Link: https://lore.kernel.org/r/tencent_4A14812842F104E93AA722EC939483CEFF05@qq.com