x86/speculation: Add virtualized speculative store bypass disable support

Some AMD processors only support a non-architectural means of enabling
speculative store bypass disable (SSBD).  To allow a simplified view of
this to a guest, an architectural definition has been created through a new
CPUID bit, 0x80000008_EBX[25], and a new MSR, 0xc001011f.  With this, a
hypervisor can virtualize the existence of this definition and provide an
architectural method for using SSBD to a guest.

Add the new CPUID feature, the new MSR and update the existing SSBD
support to use this MSR when present.

Signed-off-by: Tom Lendacky <thomas.lendacky@amd.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Borislav Petkov <bp@suse.de>