+ enable the atomics extension on the llvm-mc command line (extension appears

  to be recognized since LLVM 8.0, so it should be safe to always enable this
  from the llvm-mc command line)

compiler/systems.pas

@@ -86,7 +86,7 @@ interface
           id          : tasm;
           idtxt       : string[12];
           asmbin      : string[16];
-          asmcmd      : string[113];
+          asmcmd      : string[121];
           supported_targets : set of tsystem;
           flags        : set of tasmflags;
           labelprefix : string[3];

compiler/wasm32/agllvmmc.pas

@@ -247,7 +247,7 @@ implementation
          id     : as_wasm32_llvm_mc;
          idtxt  : 'LLVM-MC';
          asmbin : 'llvm-mc';
-         asmcmd : '--assemble --arch=wasm32 -mattr=+sign-ext,+exception-handling,+bulk-memory --filetype=obj -o $OBJ $EXTRAOPT $ASM';
+         asmcmd : '--assemble --arch=wasm32 -mattr=+sign-ext,+exception-handling,+bulk-memory,+atomics --filetype=obj -o $OBJ $EXTRAOPT $ASM';
          supported_targets : [system_wasm32_embedded,system_wasm32_wasi];
          flags : [af_smartlink_sections];
          labelprefix : '.L';