x86/cpufeatures: Add CPUID feature bit for the Bus Lock Threshold

Misbehaving guests can cause bus locks to degrade the performance of
the system. The Bus Lock Threshold feature can be used to address this
issue by providing capability to the hypervisor to limit guest's
ability to generate bus lock, thereby preventing system slowdown due
to performance penalities.

When the Bus Lock Threshold feature is enabled, the processor checks
the bus lock threshold count before executing the buslock and decides
whether to trigger bus lock exit or not.

The value of the bus lock threshold count '0' generates bus lock
exits, and if the value is greater than '0', the bus lock is executed
successfully and the bus lock threshold count is decremented.

Presence of the Bus Lock threshold feature is indicated via CPUID
function 0x8000000A_EDX[29].

Signed-off-by: Manali Shukla <manali.shukla@amd.com>
Reviewed-by: Borislav Petkov (AMD) <bp@alien8.de>
Link: https://lore.kernel.org/r/20250502050346.14274-3-manali.shukla@amd.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

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