RISC-V: Add _dl_start_user.

This is required for the debugglibc.sh script to work.  Tested by
successfully using this patched script, and a riscv64-linux testsuite
run.

We could perhaps call RTLD_EPILOGUE for ENTRY_POINT before calling
RTLD_PROLOGUE for _dl_start_user, but I don't think it matters.

OK?

Jim