vmm: Mask CPUID leaf 0x80000008 EBX on Intel hosts as well

[hayzam_gmail.com]

Summary

vmm limits 0x80000008 EBX to a small set of supported features, but
only on AMD hosts. Bare-metal Intel CPUs report at most WBNOINVD
there, but KVM on Intel reports the AMD IBPB, IBRS, STIBP and SSBD
bits (EBX=0x100d000), and bhyve passes them to its guests when it
runs under KVM. Linux treats them like the leaf 7 EDX bits and
accesses IA32_SPEC_CTRL and IA32_PRED_CMD, which vmm does not
implement and already hides in leaf 7 on Intel. An Alpine Linux
6.18 guest panics during boot with a #GP in write_ibpb().

Apply the mask on all hosts and keep the topology handling
SVM-only. This also stops advertising WBNOINVD on Intel hosts,
matching AMD.

Sponsored by: Alchemilla Ventures Private Limited

Test Plan

Xeon E-2176G with Proxmox/KVM (-cpu host), which reports CPUID
0x80000008 EBX=0x100d000 (IBPB, IBRS, STIBP, SSBD) to a FreeBSD L1.
Booted an Alpine 3.24 (Linux 6.18) guest under bhyve in the L1:

• Before: the guest panics during boot with a #GP in write_ibpb() (WRMSR to IA32_PRED_CMD). With bhyve -w it boots, but sees EBX=0x100d000 and reports IBRS mitigation although bhyve drops the MSR writes.
• After: the guest boots, sees EBX=0 and never accesses IA32_SPEC_CTRL or IA32_PRED_CMD. ECX (topology) is unchanged.

Built vmm.ko with and without BHYVE_SNAPSHOT.