generate asm symbols for target_os = "none" targets

the built-in rustc targets `{x86,x86_64,arm,aarch64}-none` all use the
ELF format

fixes briansmith#1793

Author: japaric

build.rs

@@ -260,6 +260,7 @@ const LINUX_ABI: &[&str] = &[
     "linux",
     "redox",
     "solaris",
+    "none",
 ];
 
 /// Operating systems that have the same ABI as macOS on every architecture