target/i386: Expose bits related to SRSO vulnerability

target/i386: Expose bits related to SRSO vulnerability

Add following bits related Speculative Return Stack Overflow (SRSO).
Guests can make use of these bits if supported.

These bits are reported via CPUID Fn8000_0021_EAX.
===================================================================
Bit Feature Description
===================================================================
27  SBPB                Indicates support for the Selective Branch Predictor Barrier.
28  IBPB_BRTYPE         MSR_PRED_CMD[IBPB] flushes all branch type predictions.
29  SRSO_NO             Not vulnerable to SRSO.
30  SRSO_USER_KERNEL_NO Not vulnerable to SRSO at the user-kernel boundary.
===================================================================

Link: https://www.amd.com/content/dam/amd/en/documents/corporate/cr/speculative-return-stack-overflow-whitepaper.pdf
Link: https://www.amd.com/content/dam/amd/en/documents/epyc-technical-docs/programmer-references/57238.zip
Signed-off-by: Babu Moger <babu.moger@amd.com>
Link: https://lore.kernel.org/r/dadbd70c38f4e165418d193918a3747bd715c5f4.1729807947.git.babu.moger@amd.com
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>