Use gRPC_INSTALL_LIBDIR for pkgconfig files (#29826)

* Use gRPC_INSTALL_LIBDIR for pkgconfig files

Fixes #25635.

If grpc libraries are installed in <prefix>/lib64, then .pc files should
be installed in <prefix>/lib64/pkgconfig. Before this commit, they were
always installed in <prefix>/lib/pkgconfig.

* Re-generate projects

CMakeLists.txt

@@ -20432,7 +20432,7 @@ function(generate_pkgconfig name description version requires
     "${output_filepath}"
     @ONLY)
   install(FILES "${output_filepath}"
-    DESTINATION "lib/pkgconfig/")
+    DESTINATION "${gRPC_INSTALL_LIBDIR}/pkgconfig")
 endfunction()
 
 # gpr .pc file

templates/CMakeLists.txt.template

@@ -852,7 +852,7 @@
       "<%text>${output_filepath}</%text>"
       @ONLY)
     install(FILES "<%text>${output_filepath}</%text>"
-      DESTINATION "lib/pkgconfig/")
+      DESTINATION "<%text>${gRPC_INSTALL_LIBDIR}/pkgconfig</%text>")
   endfunction()
 
   # gpr .pc file