x86/bugs: Decouple ALTERNATIVE usage from VERW macro definition

x86/bugs: Decouple ALTERNATIVE usage from VERW macro definition

Decouple the use of ALTERNATIVE from the encoding of VERW to clear CPU
buffers so that KVM can use ALTERNATIVE_2 to handle "always clear buffers"
and "clear if guest can access host MMIO" in a single statement.

No functional change intended.

Reviewed-by: Brendan Jackman <jackmanb@google.com>
Reviewed-by: Pawan Gupta <pawan.kumar.gupta@linux.intel.com>
Link: https://patch.msgid.link/20251113233746.1703361-4-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>