Support long double intrinsics in any aarch64 linux

Expands the support added in rust-lang#377 from just musl to any linux.

Fixes rust-lang#428

Author: richkadel

build.rs

@@ -425,7 +425,7 @@ mod c {
                 sources.extend(&[("__multc3", "multc3.c")]);
             }
 
-            if target_env == "musl" {
+            if target_os == "linux" {
                 sources.extend(&[
                     ("__addtf3", "addtf3.c"),
                     ("__multf3", "multf3.c"),