sim: cris: use -sim with C tests for cris-elf targets

Building the C tests with a cris-elf toolchain (gcc-10.2 &
newlib-4.1.0) currently fail due to warnings it emits:
cris-elf-ld: libc.a(lib_a-closer.o): in function `_close_r':
newlib/libc/reent/closer.c:47: warning: _close is not implemented and will always fail

This is because the default target for cris-elf is bare metal, not
the simulator.  For that, we need -sim.  So add it for elf targets.

We don't add it for all targets as the simulator (and testsuite)
run both libgloss programs as well as Linux userspace programs.