libcxx: remove no-atomics from PACKAGECONFIG

libcxx: remove no-atomics from PACKAGECONFIG

In commit [1], upstream has dropped _LIBCXXABI_HAS_ATOMIC_BUILTINS,
clean up no-atomics from PACKAGECONFIG otherwise build for qemuarmv5
failed

$ echo "MACHINE = 'qemuarmv5'" >> conf/local.conf
$ bitbake libcxx
...
| tmp/work/armv5e-oe-linux-gnueabi/libcxx/21.1.8/recipe-sysroot-native/usr/bin/arm-oe-linux-gnueabi/
arm-oe-linux-gnueabi-ld: cannot find -latomic: No such file or directory
| arm-oe-linux-gnueabi-clang++: error: linker command failed with exit code 1 (use -v to see invocation)
...

[1] https://github.com/llvm/llvm-project/commit/f8b1cc36578608f2f840d7ea8b0aa9b28fdaa492

Signed-off-by: Hongxu Jia <hongxu.jia@windriver.com>
Signed-off-by: Mathieu Dubois-Briand <mathieu.dubois-briand@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>