i386: Add "stibp" flag name

The STIBP flag may be supported by the host KVM module, so QEMU
can allow it to be configured manually, and it can be exposed to
guests when using "-cpu host".

No additional migration code is required because the whole
contents of spec_ctrl is already migrated in the "cpu/spec_ctrl"
section.

Corresponding KVM patch was submitted at:
https://lore.kernel.org/lkml/20181205191956.31480-1-ehabkost@redhat.com/Signed-off-by: NEduardo Habkost <ehabkost@redhat.com>
Message-Id: <20181210180250.31299-1-ehabkost@redhat.com>
Signed-off-by: NEduardo Habkost <ehabkost@redhat.com>

target/i386/cpu.c

@@ -1042,7 +1042,7 @@ static FeatureWordInfo feature_word_info[FEATURE_WORDS] = {
             NULL, NULL, NULL, NULL,
             NULL, NULL, "pconfig", NULL,
             NULL, NULL, NULL, NULL,
-            NULL, NULL, "spec-ctrl", NULL,
+            NULL, NULL, "spec-ctrl", "stibp",
             NULL, "arch-capabilities", NULL, "ssbd",
         },
         .cpuid = {