Support SDKROOT env var on iOS

Following what clang does (https://github.com/llvm/llvm-project/blob/296a80102a9b72c3eda80558fb78a3ed8849b341/clang/lib/Driver/ToolChains/Darwin.cpp#L1661-L1678), allow allow SDKROOT to tell us where the Apple SDK lives so we don't have to invoke xcrun.

Author: swolchok

Diff for: src/librustc_target/spec/apple_ios_base.rs

@@ -1,3 +1,4 @@
+use std::env;
 use std::io;
 use std::process::Command;
 use crate::spec::{LinkArgs, LinkerFlavor, TargetOptions};
@@ -27,6 +28,12 @@ impl Arch {
 }
 
 pub fn get_sdk_root(sdk_name: &str) -> Result<String, String> {
+    if let Some(sdkroot) = env::var("SDKROOT").ok() {
+        let sdkroot_path = Path::new(sdkroot);
+        if sdkroot_path.is_absolute() && sdkroot_path != Path::new("/") && sdkroot_path.exists() {
+            return Ok(sdkroot);
+        }
+    }
     let res = Command::new("xcrun")
                       .arg("--show-sdk-path")
                       .arg("-sdk")