rustc: Link to Android ABI requirements.

Hopefully these references will be stable and provide guidance
when requirements change in the future.

Author: rillian

src/librustc_back/target/aarch64_linux_android.rs

@@ -10,6 +10,9 @@
 
 use target::{Target, TargetOptions, TargetResult};
 
+// See https://developer.android.com/ndk/guides/abis.html#arm64-v8a
+// for target ABI requirements.
+
 pub fn target() -> TargetResult {
     let mut base = super::android_base::opts();
     base.max_atomic_width = Some(128);

src/librustc_back/target/armv7_linux_androideabi.rs

@@ -10,6 +10,9 @@
 
 use target::{Target, TargetOptions, TargetResult};
 
+// See https://developer.android.com/ndk/guides/abis.html#v7a
+// for target ABI requirements.
+
 pub fn target() -> TargetResult {
     let mut base = super::android_base::opts();
     base.features = "+v7,+thumb2,+vfp3,+d16,-neon".to_string();

src/librustc_back/target/i686_linux_android.rs

@@ -10,6 +10,9 @@
 
 use target::{Target, TargetResult};
 
+// See https://developer.android.com/ndk/guides/abis.html#x86
+// for target ABI requirements.
+
 pub fn target() -> TargetResult {
     let mut base = super::android_base::opts();