tests/tcg: ensure s390x-softmmu output redirected

The multiarch system tests output serial data which should be
redirected to the "output" chardev rather than echoed to the console.

Comment the use of EXTFLAGS variable while we are at it.

Acked-by: Ilya Leoshkevich <iii@linux.ibm.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20240916085400.1046925-13-alex.bennee@linaro.org>

tests/tcg/s390x/Makefile.softmmu-target

@@ -1,6 +1,7 @@
 S390X_SRC=$(SRC_PATH)/tests/tcg/s390x
 VPATH+=$(S390X_SRC)
-QEMU_OPTS+=-action panic=exit-failure -nographic $(EXTFLAGS) -kernel
+# EXTFLAGS can be passed by the user, e.g. to override the --accel
+QEMU_OPTS+=-action panic=exit-failure -nographic -serial chardev:output $(EXTFLAGS) -kernel
 LINK_SCRIPT=$(S390X_SRC)/softmmu.ld
 CFLAGS+=-ggdb -O0
 LDFLAGS=-nostdlib -static