rustc_target: Move target_os from Target to TargetOptions

compiler/rustc_target/src/spec/android_base.rs

@@ -2,6 +2,7 @@ use crate::spec::{LinkerFlavor, TargetOptions};
 
 pub fn opts() -> TargetOptions {
     let mut base = super::linux_base::opts();
+    base.target_os = "android".to_string();
     // Many of the symbols defined in compiler-rt are also defined in libgcc.
     // Android's linker doesn't like that by default.
     base.pre_link_args