hppa: Fix initialization of dp register [BZ 29635]

After upgrading glibc to Debian 2.35-1, gdb faulted on
startup and dropped core in a function call in the main
application.  This was caused by not initializing the
global dp register for the main application early enough.

Restore the code to initialize dp in _dl_start_user.
It was removed when code was added to initialize dp in
elf_machine_runtime_setup.

Signed-off-by: John David Anglin <dave.anglin@bell.net>