AIX: Fix executable ENABLE_EXPORTS in Makefile generators

In commit afcd9fe669 (AIX: Add an option to disable automatic exports
from shared libraries, 2020-01-30, v3.17.0-rc1~47^2) the population of
the `<AIX_EXPORTS>` rule variable placeholder was accidentally added to
the device linking rule rather than the main linking rule.  This caused
our `ExportImportList` script on AIX, when called for executables with
`ENABLE_EXPORTS` set, to be given an `AIX_EXPORTS` file name that does
not exist, leading to a warning from the `dump` tool.

Move the population of the `<AIX_EXPORTS>` placeholder in the Makefile
generators to the main link rule.

Issue: #20290

Source/cmMakefileExecutableTargetGenerator.cxx

@@ -195,8 +195,6 @@ void cmMakefileExecutableTargetGenerator::WriteNvidiaDeviceExecutableRule(
     this->CreateObjectLists(useLinkScript, false, useResponseFileForObjects,
                             buildObjs, depends, useWatcomQuote);
 
-    std::string const& aixExports = this->GetAIXExports(this->GetConfigName());
-
     cmRulePlaceholderExpander::RuleVariables vars;
     std::string objectDir = this->GeneratorTarget->GetSupportDirectory();
 
@@ -217,7 +215,6 @@ void cmMakefileExecutableTargetGenerator::WriteNvidiaDeviceExecutableRule(
                                                   cmOutputConverter::SHELL);
 
     vars.Language = linkLanguage.c_str();
-    vars.AIXExports = aixExports.c_str();
     vars.Objects = buildObjs.c_str();
     vars.ObjectDir = objectDir.c_str();
     vars.Target = target.c_str();
@@ -528,11 +525,14 @@ void cmMakefileExecutableTargetGenerator::WriteExecutableRule(bool relink)
 
     std::string manifests = this->GetManifests(this->GetConfigName());
 
+    std::string const& aixExports = this->GetAIXExports(this->GetConfigName());
+
     cmRulePlaceholderExpander::RuleVariables vars;
     vars.CMTargetName = this->GeneratorTarget->GetName().c_str();
     vars.CMTargetType =
       cmState::GetTargetTypeName(this->GeneratorTarget->GetType()).c_str();
     vars.Language = linkLanguage.c_str();
+    vars.AIXExports = aixExports.c_str();
     vars.Objects = buildObjs.c_str();
     std::string objectDir = this->GeneratorTarget->GetSupportDirectory();