Use capstone to validate precise-output tests (#5780)

Use the capstone library to disassemble precise-output tests, in addition to pretty-printing their vcode.

cranelift/codegen/src/isa/x64/mod.rs

@@ -84,7 +84,7 @@ impl TargetIsa for X64Backend {
         Ok(CompiledCodeStencil {
             buffer,
             frame_size,
-            disasm: emit_result.disasm,
+            vcode: emit_result.disasm,
             value_labels_ranges,
             sized_stackslot_offsets,
             dynamic_stackslot_offsets,
@@ -171,6 +171,16 @@ impl TargetIsa for X64Backend {
     fn function_alignment(&self) -> u32 {
         16
     }
+
+    #[cfg(feature = "disas")]
+    fn to_capstone(&self) -> Result<capstone::Capstone, capstone::Error> {
+        use capstone::prelude::*;
+        Capstone::new()
+            .x86()
+            .mode(arch::x86::ArchMode::Mode64)
+            .syntax(arch::x86::ArchSyntax::Att)
+            .build()
+    }
 }
 
 impl fmt::Display for X64Backend {