CVE-2024-31142 x86: Incorrect logic for BTC/SRSO mitigations

Because of a logical error in XSA-407 Branch Type Confusion, the mitigation is not applied properly when it is intended to be used. XSA-434 Speculative Return Stack Overflow uses the same infrastructure, so is equally impacted. For more details, see: https://xenbits.xen.org/xsa/advisory-407.html...