Add Xtensa as an experimental target

compiler/rustc_llvm/src/lib.rs

@@ -188,6 +188,13 @@ pub fn initialize_available_targets() {
         LLVMInitializeHexagonAsmPrinter,
         LLVMInitializeHexagonAsmParser
     );
+    init_target!(
+        llvm_component = "xtensa",
+        LLVMInitializeXtensaTargetInfo,
+        LLVMInitializeXtensaTarget,
+        LLVMInitializeXtensaTargetMC,
+        LLVMInitializeXtensaAsmParser
+    );
     init_target!(
         llvm_component = "webassembly",
         LLVMInitializeWebAssemblyTargetInfo,

src/bootstrap/src/core/build_steps/llvm.rs

@@ -330,7 +330,7 @@ impl Step for Llvm {
 
         let llvm_exp_targets = match builder.config.llvm_experimental_targets {
             Some(ref s) => s,
-            None => "AVR;M68k;CSKY",
+            None => "AVR;M68k;CSKY;Xtensa",
         };
 
         let assertions = if builder.config.llvm_assertions { "ON" } else { "OFF" };