[libc++] Revert <stdatomic.h> changes

This reverts commits a30a7948d and 5d1c1a243, which broke the LLDB
data formatters tests because they build with modules in C++11 mode.

Differential Revision: https://reviews.llvm.org/D97044

NOKEYCHECK=True
GitOrigin-RevId: 987c7f407d1414a023f03eb788bb97667b479f27
diff --git a/docs/FeatureTestMacroTable.rst b/docs/FeatureTestMacroTable.rst
index a6c658c..8877ec0 100644
--- a/docs/FeatureTestMacroTable.rst
+++ b/docs/FeatureTestMacroTable.rst
@@ -342,7 +342,7 @@
     ------------------------------------------------- -----------------
     ``__cpp_lib_stacktrace``                          *unimplemented*
     ------------------------------------------------- -----------------
-    ``__cpp_lib_stdatomic_h``                         ``202011L``
+    ``__cpp_lib_stdatomic_h``                         *unimplemented*
     ------------------------------------------------- -----------------
     ``__cpp_lib_string_contains``                     ``202011L``
     ------------------------------------------------- -----------------