riscv: vdso: Only try to install vDSO when present

riscv: vdso: Only try to install vDSO when present

vdso.so.dbg is only built with CONFIG_MMU.

Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202607090258.iSAUYlO1-lkp@intel.com/
Fixes: f157d411a9eb ("riscv: add missing vdso_install target")
Fixes: 3edf39916977 ("vDSO, kbuild: Provide vDSO debug variants at runtime")
Signed-off-by: Thomas Weißschuh <thomas.weissschuh@linutronix.de>
Link: https://patch.msgid.link/20260709-riscv-install-vdso-v1-1-0ba4345419ca@linutronix.de
Signed-off-by: Paul Walmsley <pjw@kernel.org>