serial: nsim: Fix impossible-to-enable CONFIG_UART_NSIM

CONFIG_UART_NSIM depends on CONFIG_NSIM, which was removed in commit
9bc69a46 ("boards: Update arc em_starterkit support from 2.2 to
2.3"). Remove the dependency, and also remove the CONFIG_NSIM=y setting
from the test_nsim test (which should now work).

Also change the condition for EXTERN()ing _VectorTable in
include/arch/arc/v2/linker.ld to check CONFIG_UART_NSIM instead of
CONFIG_NSIM. I'm guessing the EXTERN() is there to make the symbol
visible to nSIM, though I don't know anything about it.

Signed-off-by: Ulf Magnusson <Ulf.Magnusson@nordicsemi.no>