Log Message:
Always create a ${TOOLS_DIR}/bin/rpcgen to wrap the real rpcgen.
The wrapper will correctly set the CPP environment variable to a
stat((2)able path to a C preprocessor, then rely on the PATH to
find and invoke the real rpcgen.

Remove NO_EXPORT_CPP in package Makefiles where it was used just to
avoid problems with rpcgen.  The build system now just does the right
thing automatically without needing package-specific knowledge.

This fixes PR pkg/27272.