Add a wasmtime settings command to print Cranelift settings.

This commit adds the `wasmtime settings` command to print out available
Cranelift settings for a target (defaults to the host).

The compile command has been updated to remove the Cranelift ISA options in
favor of encouraging users to use `wasmtime settings` to discover what settings
are available.  This will reduce the maintenance cost for syncing the compile
command with Cranelift ISA flags.

cranelift/codegen/meta/src/cdsl/settings.rs

@@ -20,6 +20,7 @@ pub(crate) enum SpecificSetting {
 #[derive(Hash, PartialEq, Eq)]
 pub(crate) struct Setting {
     pub name: &'static str,
+    pub description: &'static str,
     pub comment: &'static str,
     pub specific: SpecificSetting,
     pub byte_offset: u8,
@@ -88,6 +89,7 @@ impl Into<PresetType> for PresetIndex {
 #[derive(Hash, PartialEq, Eq)]
 pub(crate) struct Preset {
     pub name: &'static str,
+    pub description: &'static str,
     values: Vec<BoolSettingIndex>,
 }
 
@@ -169,6 +171,7 @@ pub(crate) enum ProtoSpecificSetting {
 /// This is the information provided during building for a setting.
 struct ProtoSetting {
     name: &'static str,
+    description: &'static str,
     comment: &'static str,
     specific: ProtoSpecificSetting,
 }
@@ -251,11 +254,13 @@ impl SettingGroupBuilder {
     fn add_setting(
         &mut self,
         name: &'static str,
+        description: &'static str,
         comment: &'static str,
         specific: ProtoSpecificSetting,
     ) {
         self.settings.push(ProtoSetting {
             name,
+            description,
             comment,
             specific,
         })
@@ -264,6 +269,7 @@ impl SettingGroupBuilder {
     pub fn add_bool(
         &mut self,
         name: &'static str,
+        description: &'static str,
         comment: &'static str,
         default: bool,
     ) -> BoolSettingIndex {
@@ -271,28 +277,55 @@ impl SettingGroupBuilder {
             self.predicates.is_empty(),
             "predicates must be added after the boolean settings"
         );
-        self.add_setting(name, comment, ProtoSpecificSetting::Bool(default));
+        self.add_setting(
+            name,
+            description,
+            comment,
+            ProtoSpecificSetting::Bool(default),
+        );
         BoolSettingIndex(self.settings.len() - 1)
     }
 
     pub fn add_enum(
         &mut self,
         name: &'static str,
+        description: &'static str,
         comment: &'static str,
         values: Vec<&'static str>,
     ) {
-        self.add_setting(name, comment, ProtoSpecificSetting::Enum(values));
+        self.add_setting(
+            name,
+            description,
+            comment,
+            ProtoSpecificSetting::Enum(values),
+        );
     }
 
-    pub fn add_num(&mut self, name: &'static str, comment: &'static str, default: u8) {
-        self.add_setting(name, comment, ProtoSpecificSetting::Num(default));
+    pub fn add_num(
+        &mut self,
+        name: &'static str,
+        description: &'static str,
+        comment: &'static str,
+        default: u8,
+    ) {
+        self.add_setting(
+            name,
+            description,
+            comment,
+            ProtoSpecificSetting::Num(default),
+        );
     }
 
     pub fn add_predicate(&mut self, name: &'static str, node: PredicateNode) {
         self.predicates.push(ProtoPredicate { name, node });
     }
 
-    pub fn add_preset(&mut self, name: &'static str, args: Vec<PresetType>) -> PresetIndex {
+    pub fn add_preset(
+        &mut self,
+        name: &'static str,
+        description: &'static str,
+        args: Vec<PresetType>,
+    ) -> PresetIndex {
         let mut values = Vec::new();
         for arg in args {
             match arg {
@@ -302,7 +335,11 @@ impl SettingGroupBuilder {
                 PresetType::BoolSetting(index) => values.push(index),
             }
         }
-        self.presets.push(Preset { name, values });
+        self.presets.push(Preset {
+            name,
+            description,
+            values,
+        });
         PresetIndex(self.presets.len() - 1)
     }
 
@@ -347,6 +384,7 @@ impl SettingGroupBuilder {
 
             group.settings.push(Setting {
                 name: s.name,
+                description: s.description,
                 comment: s.comment,
                 byte_offset,
                 specific,
@@ -367,6 +405,7 @@ impl SettingGroupBuilder {
             };
             group.settings.push(Setting {
                 name: s.name,
+                description: s.description,
                 comment: s.comment,
                 byte_offset: byte_offset + predicate_number / 8,
                 specific: SpecificSetting::Bool(BoolSetting {