rustc_llvm/build.rs: improve comment for NetBSD/i386 targets

...explaining why we need -latomic (gcc & g++ built for i486,
and LLVM insisting on use of 64-bit atomics).

compiler/rustc_llvm/build.rs

@@ -258,7 +258,9 @@ fn main() {
     {
         println!("cargo:rustc-link-lib=z");
     } else if target.contains("netbsd") {
-        // Building for i586 or i686, we need -latomic for 64-bit atomics
+        // On NetBSD/i386, gcc and g++ is built for i486 (to maximize backward compat)
+        // However, LLVM insists on using 64-bit atomics.
+        // This gives rise to a need to link rust itself with -latomic for these targets
         if target.starts_with("i586")
            || target.starts_with("i686")
         {