Explain why libatomic is not needed on FreeBSD riscv64

From Jessica Clarke (jrtc27@)

compiler/rustc_llvm/build.rs

@@ -276,7 +276,10 @@ fn main() {
         "stdc++"
     };
 
-    // RISC-V requires libatomic for sub-word atomic operations
+    // RISC-V GCC erroneously requires libatomic for sub-word
+    // atomic operations. FreeBSD uses Clang as its system
+    // compiler and provides no libatomic in its base system so
+    // does not want this.
     if !target.contains("freebsd") && target.starts_with("riscv") {
         println!("cargo:rustc-link-lib=atomic");
     }

src/bootstrap/native.rs

@@ -250,8 +250,13 @@ impl Step for Llvm {
         }
 
         if !target.contains("freebsd") && target.starts_with("riscv") {
-            // In RISC-V, using C++ atomics require linking to `libatomic` but the LLVM build
-            // system check cannot detect this. Therefore it is set manually here.
+            // RISC-V GCC erroneously requires linking against
+            // `libatomic` when using 1-byte and 2-byte C++
+            // atomics but the LLVM build system check cannot
+            // detect this. Therefore it is set manually here.
+            // FreeBSD uses Clang as its system compiler and
+            // provides no libatomic in its base system so does
+            // not want this.
             if !builder.config.llvm_tools_enabled {
                 cfg.define("CMAKE_EXE_LINKER_FLAGS", "-latomic");
             } else {