x86/opcode: Add ERET[US] instructions to the x86 opcode map

ERETU returns from an event handler while making a transition to ring 3,
and ERETS returns from an event handler while staying in ring 0.

Add instruction opcodes used by ERET[US] to the x86 opcode map; opcode
numbers are per FRED spec v5.0.

Signed-off-by: H. Peter Anvin (Intel) <hpa@zytor.com>
Signed-off-by: Xin Li <xin3.li@intel.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Tested-by: Shan Kang <shan.kang@intel.com>
Reviewed-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
Link: https://lore.kernel.org/r/20231205105030.8698-10-xin3.li@intel.com