CVE-2024-35996: cpu: Re-enable CPU mitigations by default for !X86 architectures

In the Linux kernel, the following vulnerability has been resolved:

cpu: Re-enable CPU mitigations by default for !X86 architectures

Rename x86's to CPUMITIGATIONS, define it in generic code, and force it on for all architectures exception x86. A recent commit to turn mitigations off by default if SPECULATIONMITIGATIONS=n kinda sorta missed that "cpumitigations" is completely generic, whereas SPECULATIONMITIGATIONS is x86-specific.

Rename x86's SPECULATIVEMITIGATIONS instead of keeping both and have it select CPUMITIGATIONS, as having two configs for the same thing is unnecessary and confusing. This will also allow x86 to use the knob to manage mitigations that aren't strictly related to speculative execution.

Use another Kconfig to communicate to common code that CPUMITIGATIONS is already defined instead of having x86's menu depend on the common CPUMITIGATIONS. This allows keeping a single point of contact for all of x86's mitigations, and it's not clear that other architectures want to allow disabling mitigations at compile-time.