gen_settings: dont try to display a Preset descriptor in Flags (#241)

* gen_settings: dont try to display a Preset descriptor in Flags

Trying to display a preset doesnt make sense, and before this commit it
does not display anything meaningful - the printout just says e.g.
"haswell =\n".

The offset byte a preset descriptor isnt a valid offset into the
flag bytes, it is actually an offset into the PRESETS table. It will
cause a panic when the offset is out of bounds for the flag bytes,
which happens in the intel isa as of this commit.

* intel settings: test that display impl doesnt panic

lib/cretonne/src/settings.rs

@@ -297,6 +297,17 @@ pub mod detail {
         /// The `Descriptor::offset` field refers to the `PRESETS` table.
         Preset,
     }
+
+    impl Detail {
+        /// Check if a detail is a Detail::Preset. Useful because the Descriptor
+        /// offset field has a different meaning when the detail is a preset.
+        pub fn is_preset(&self) -> bool {
+            match self {
+                &Detail::Preset => true,
+                _ => false,
+            }
+        }
+    }
 }
 
 // Include code generated by `meta/gen_settings.py`. This file contains a public `Flags` struct