AIX: Add an option to disable automatic exports from shared libraries

Since commit 0f150b69d3 (AIX: Explicitly compute shared object exports
for both XL and GNU, 2019-07-11, v3.16.0-rc1~418^2~2) we always export
all symbols from shared libraries by default.  Add a new target property
called `AIX_EXPORT_ALL_SYMBOLS` that can be explicitly set to OFF to
suppress this behavior and export no symbols by default.

Fixes: #20290

Source/cmCommonTargetGenerator.cxx

@@ -17,6 +17,7 @@
 #include "cmSourceFile.h"
 #include "cmStateTypes.h"
 #include "cmStringAlgorithms.h"
+#include "cmTarget.h"
 
 cmCommonTargetGenerator::cmCommonTargetGenerator(cmGeneratorTarget* gt)
   : GeneratorTarget(gt)
@@ -216,6 +217,20 @@ std::string cmCommonTargetGenerator::GetManifests(const std::string& config)
   return cmJoin(manifests, " ");
 }
 
+std::string cmCommonTargetGenerator::GetAIXExports(std::string const&)
+{
+  std::string aixExports;
+  if (this->GeneratorTarget->Target->IsAIX()) {
+    if (const char* exportAll =
+          this->GeneratorTarget->GetProperty("AIX_EXPORT_ALL_SYMBOLS")) {
+      if (cmIsOff(exportAll)) {
+        aixExports = "-n";
+      }
+    }
+  }
+  return aixExports;
+}
+
 void cmCommonTargetGenerator::AppendOSXVerFlag(std::string& flags,
                                                const std::string& lang,
                                                const char* name, bool so)