KVM: SVM: Wrap __svm_sev_es_vcpu_run() with #ifdef CONFIG_KVM_AMD_SEV

KVM: SVM: Wrap __svm_sev_es_vcpu_run() with #ifdef CONFIG_KVM_AMD_SEV

Compile (and link) __svm_sev_es_vcpu_run() if and only if SEV support is
actually enabled.  This will allow dropping non-existent 32-bit "support"
from __svm_sev_es_vcpu_run() without causing undue confusion.

Intentionally don't provide a stub (but keep the declaration), as any sane
compiler, even with things like KASAN enabled, should eliminate the call
to __svm_sev_es_vcpu_run() since sev_es_guest() unconditionally returns
"false" if CONFIG_KVM_AMD_SEV=n.

Reviewed-by: Tom Lendacky <thomas.lendacky@amd.com>
Link: https://lore.kernel.org/r/20240223204233.3337324-3-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>