[dv/uvm/core_ibex] Pass ISA from Makefile

This allows run.py to pass ISA options through to Spike. This relates to
the failure in #1369 (test inserts a bitmanip instruction which Spike
interprets as valid but the core treats as invalid depending on config).

Signed-off-by: Tom Roberts <tomroberts@lowrisc.org>

dv/uvm/core_ibex/Makefile

@@ -318,6 +318,7 @@ $(metadata)/instr_gen.iss.stamp: \
      ${TEST_OPTS} \
      --iss="${ISS}" \
      --iss_opts="${ISS_OPTS}" \
+     --isa="${ISA}" \
      ${RISCV_DV_OPTS}
 	$(call dump-vars,$(metadata)/iss-vars.mk,iss,$(iss-var-deps))
 	@touch $@