cmake: fix cross-compilation with gRPC_BUILD_GRPC_CPP_PLUGIN=OFF (#26292)

* cmake: fix cross-compilation with gRPC_BUILD_GRPC_CPP_PLUGIN=OFF

When cross-compiling gRPC, a _native_ version of 'grpc_cpp_plugin' is
searched in the environment. For most use cases, a _cross_ version of this
file is not needed and gRPC_BUILD_GRPC_CPP_PLUGIN can be set to OFF.

However, when cross-building with -DgRPC_BUILD_GRPC_CPP_PLUGIN=OFF, there
are some build errors, for example:

make[3]: *** No rule to make target 'grpc_cpp_plugin', needed by 'gens/src/proto/grpc/reflection/v1alpha/reflection.grpc.pb.cc'.  Stop.

This is because there is still a hard dependency on 'grpc_cpp_plugin' for
these targets, not taking into account the cross-compilation case.

Fix by depending on the variable gRPC_CPP_PLUGIN, which is set correctly for
either cross or native case.

* regenerate projects

CMakeLists.txt

@@ -325,7 +325,7 @@ function(protobuf_generate_grpc_cpp)
            --plugin=protoc-gen-grpc=${_gRPC_CPP_PLUGIN}
            ${_protobuf_include_path}
            ${REL_FIL}
-      DEPENDS ${ABS_FIL} ${_gRPC_PROTOBUF_PROTOC} grpc_cpp_plugin
+      DEPENDS ${ABS_FIL} ${_gRPC_PROTOBUF_PROTOC} ${_gRPC_CPP_PLUGIN}
       WORKING_DIRECTORY ${CMAKE_CURRENT_SOURCE_DIR}
       COMMENT "Running gRPC C++ protocol buffer compiler on ${FIL}"
       VERBATIM)

templates/CMakeLists.txt.template

@@ -395,7 +395,7 @@
              --plugin=protoc-gen-grpc=<%text>${_gRPC_CPP_PLUGIN}</%text>
              <%text>${_protobuf_include_path}</%text>
              <%text>${REL_FIL}</%text>
-        DEPENDS <%text>${ABS_FIL}</%text> <%text>${_gRPC_PROTOBUF_PROTOC}</%text> grpc_cpp_plugin
+        DEPENDS <%text>${ABS_FIL}</%text> <%text>${_gRPC_PROTOBUF_PROTOC}</%text> <%text>${_gRPC_CPP_PLUGIN}</%text>
         WORKING_DIRECTORY <%text>${CMAKE_CURRENT_SOURCE_DIR}</%text>
         COMMENT "Running gRPC C++ protocol buffer compiler on <%text>${FIL}</%text>"
         VERBATIM)