contrib/plugins: add Darwin support

Under Darwin, using -shared makes it impossible to have undefined symbols
and -bundle has to be used instead; so detect the OS and use
different options.

Based-on: <20230907101811.469236-1-pbonzini@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

contrib/plugins/Makefile

@@ -37,7 +37,11 @@ all: $(SONAMES)
 	$(CC) $(CFLAGS) -c -o $@ $<
 
 lib%.so: %.o
+ifeq ($(CONFIG_DARWIN),y)
+	$(CC) -bundle -Wl,-undefined,dynamic_lookup -o $@ $^ $(LDLIBS)
+else
 	$(CC) -shared -o $@ $^ $(LDLIBS)
+endif
 
 clean:
 	rm -f *.o *.so *.d