Correct the LLVM target of aarch64-unknown-uefi

As noticed in [1], the LLVM target string  for aarch64 UEFI is not
correct. Fix it here.

Link: #132975 (comment) [1]

Author: tgross35

compiler/rustc_target/src/spec/targets/aarch64_unknown_uefi.rs

@@ -11,7 +11,7 @@ pub(crate) fn target() -> Target {
     base.features = "+v8a".into();
 
     Target {
-        llvm_target: "aarch64-unknown-windows".into(),
+        llvm_target: "aarch64-unknown-uefi".into(),
         metadata: crate::spec::TargetMetadata {
             description: Some("ARM64 UEFI".into()),
             tier: Some(2),