AIX: Fix INSTALL_REMOVE_ENVIRONMENT_RPATH with GNU compiler

Since commit baa33acbda (AIX-GNU: Put implicit link directories in
runtime libpath, 2013-02-14, v2.8.11~184^2~1) we explicitly add the GNU
compiler driver's builtin RPATH entries.  Extend commit f08dcbffec
(Property: Add INSTALL_REMOVE_ENVIRONMENT_RPATH property, 2019-07-12,
v3.16.0-rc1~350^2~1) to exclude them.

Fixes: #27738

Source/cmComputeLinkInformation.cxx

@@ -2294,7 +2294,11 @@ void cmComputeLinkInformation::GetRPath(std::vector<std::string>& runtimeDirs,
 
   // Add runtime paths required by the toolchains' runtime libraries.
   // This is done even when skipping RPATH entries modeled by CMake.
-  {
+  // However, INSTALL_REMOVE_ENVIRONMENT_RPATH tells us to exclude
+  // toolchain-defined RPATH entries from installation.
+  bool const removeEnvironmentRPath = for_install &&
+    this->Target->GetPropertyAsBool("INSTALL_REMOVE_ENVIRONMENT_RPATH");
+  if (!removeEnvironmentRPath) {
     cmGeneratorTarget::LinkClosure const* lc =
       this->Target->GetLinkClosure(this->Config);
     for (std::string const& li : lc->Languages) {