target-i386: Fix build by providing stub kvm_arch_get_supported_cpuid()

Fix build failures with clang when KVM is not enabled by
providing a stub version of kvm_arch_get_supported_cpuid().
We retain the compile time check that this function isn't
called when CONFIG_KVM is not set by guarding the stub with
ifndef __OPTIMIZE__ (we assume that an optimizing build will
do sufficient constant folding and dead code elimination to
remove the calls before linking).

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

target-i386/kvm-stub.c

@@ -16,3 +16,15 @@ bool kvm_allows_irq0_override(void)
 {
     return 1;
 }
+
+#ifndef __OPTIMIZE__
+/* This function is only called inside conditionals which we
+ * rely on the compiler to optimize out when CONFIG_KVM is not
+ * defined.
+ */
+uint32_t kvm_arch_get_supported_cpuid(KVMState *env, uint32_t function,
+                                      uint32_t index, int reg)
+{
+    abort();
+}
+#endif