sim: always enable support for the --endian option

sim: always enable support for the --endian option

The gdb code always passes down -E <little|big> to the sim core when using
the sim target.  But the sim core only recognizes this option when the sim
supports big endian systems.  So for little endian simulators, any attempt
to use the sim target fails with:

(gdb) target sim
gdbsim: invalid option -- 'E'
unable to create simulator instance

Since always respecting the option doesn't cause any problems, do just
that.  If someone tries to use an invalid endian, they'll get an error
anyways.