x86/cpufeatures: Add a CPU feature bit for MSR immediate form instructions

x86/cpufeatures: Add a CPU feature bit for MSR immediate form instructions

The immediate form of MSR access instructions are primarily motivated
by performance, not code size: by having the MSR number in an immediate,
it is available *much* earlier in the pipeline, which allows the
hardware much more leeway about how a particular MSR is handled.

Use a scattered CPU feature bit for MSR immediate form instructions.

Suggested-by: Borislav Petkov (AMD) <bp@alien8.de>
Signed-off-by: Xin Li (Intel) <xin@zytor.com>
Link: https://lore.kernel.org/r/20250805202224.1475590-2-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>