cmake: emu: nsim: Allow all nsim platform use mdb

Currently the default runner of nsim_hs_smp is mdb,
the runner of other nsim platform is nsimdrv, user
can't change the default runner unless he uses
west command(--runner).

With this change cmake will choose runner according
to BOARD_DEBUG_RUNNER or BOARD_FLASH_RUNNER. so if
add -DBOARD_FLASH_RUNNER=mdb-nsim or
-DBOARD_DEBUG_RUNNER=mdb-nsim to cmake command,
the apllication will be run with mdb.

Signed-off-by: Jingru Wang <jingru@synopsys.com>