Generate an iOS LLVM target with a specific version

Without the specific version, the mach-o header will be missing the
minimum supported operating system version. This is mandatory for
running Rust binaries on iOS devices.

compiler/rustc_target/src/spec/aarch64_apple_ios.rs

@@ -2,8 +2,15 @@ use super::apple_sdk_base::{opts, Arch};
 use crate::spec::{FramePointer, Target, TargetOptions};
 
 pub fn target() -> Target {
+    // Clang automatically chooses a more specific target based on
+    // IPHONEOS_DEPLOYMENT_TARGET.
+    // This is required for the target to pick the right
+    // MACH-O commands, so we do too.
+    let arch = "arm64";
+    let llvm_target = super::apple_base::ios_llvm_target(arch);
+
     Target {
-        llvm_target: "arm64-apple-ios".to_string(),
+        llvm_target,
         pointer_width: 64,
         data_layout: "e-m:o-i64:64-i128:128-n32:64-S128".to_string(),
         arch: "aarch64".to_string(),

compiler/rustc_target/src/spec/apple_base.rs

@@ -91,6 +91,11 @@ fn ios_deployment_target() -> (u32, u32) {
     deployment_target("IPHONEOS_DEPLOYMENT_TARGET").unwrap_or((7, 0))
 }
 
+pub fn ios_llvm_target(arch: &str) -> String {
+    let (major, minor) = ios_deployment_target();
+    format!("{}-apple-ios{}.{}.0", arch, major, minor)
+}
+
 pub fn ios_sim_llvm_target(arch: &str) -> String {
     let (major, minor) = ios_deployment_target();
     format!("{}-apple-ios{}.{}.0-simulator", arch, major, minor)