cranelift-codegen: move old x86 and RISC-V backends to isa/legacy/.

These backends will be removed in the future (see
bytecodealliance/rfcs#12 and the pending #3009 in this repo).

In the meantime, to more clearly communicate that they are using
"legacy" APIs and will eventually be removed, this PR places them in an
`isa/legacy/` subdirectory. No functional changes otherwise.

cranelift/codegen/src/isa/legacy/riscv/settings.rs

@@ -0,0 +1,56 @@
+//! RISC-V Settings.
+
+use crate::settings::{self, detail, Builder, Value};
+use core::fmt;
+
+// Include code generated by `cranelift-codegen/meta/src/gen_settings.rs`. This file contains a
+// public `Flags` struct with an impl for all of the settings defined in
+// `cranelift-codegen/meta/src/isa/riscv/mod.rs`.
+include!(concat!(env!("OUT_DIR"), "/settings-riscv.rs"));
+
+#[cfg(test)]
+mod tests {
+    use super::{builder, Flags};
+    use crate::settings::{self, Configurable};
+    use alloc::string::ToString;
+
+    #[test]
+    fn display_default() {
+        let shared = settings::Flags::new(settings::builder());
+        let b = builder();
+        let f = Flags::new(&shared, b);
+        assert_eq!(
+            f.to_string(),
+            "[riscv]\n\
+             supports_m = false\n\
+             supports_a = false\n\
+             supports_f = false\n\
+             supports_d = false\n\
+             enable_m = true\n\
+             enable_e = false\n"
+        );
+        // Predicates are not part of the Display output.
+        assert_eq!(f.full_float(), false);
+    }
+
+    #[test]
+    fn predicates() {
+        let mut sb = settings::builder();
+        sb.set("enable_simd", "true").unwrap();
+        let shared = settings::Flags::new(sb);
+        let mut b = builder();
+        b.enable("supports_f").unwrap();
+        b.enable("supports_d").unwrap();
+        let f = Flags::new(&shared, b);
+        assert_eq!(f.full_float(), true);
+
+        let mut sb = settings::builder();
+        sb.set("enable_simd", "false").unwrap();
+        let shared = settings::Flags::new(sb);
+        let mut b = builder();
+        b.enable("supports_f").unwrap();
+        b.enable("supports_d").unwrap();
+        let f = Flags::new(&shared, b);
+        assert_eq!(f.full_float(), false);
+    }
+}