gnu: cross-gcc: Fix build of 'gcc-cross-sans-libc'.

* gnu/packages/cross-base.scm (cross-gcc): Add "--disable-libatomic".

gnu/packages/cross-base.scm

@@ -101,6 +101,7 @@ GCC that does not target a libc; otherwise, target that libc."
                                    "--enable-languages=c"
 
                                    "--disable-threads" ; libgcc, would need libc
+                                   "--disable-libatomic"
                                    "--disable-libmudflap"
                                    "--disable-libgomp"
                                    "--disable-libssp"