tests/tcg: add some help output for running individual tests

So you can do:

  cd tests/tcg/aarch64-linux-user
  make -f ../Makefile.target help

To see the list of tests. You can then run each one individually.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Acked-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Message-Id: <20230315174331.2959-8-alex.bennee@linaro.org>

tests/tcg/Makefile.target

@@ -201,3 +201,10 @@ clean:
 
 distclean:
 	rm -f config-cc.mak config-target.mak ../config-$(TARGET).mak
+
+.PHONY: help
+help:
+	@echo "TCG tests help $(TARGET_NAME)"
+	@echo "Built with $(CC)"
+	@echo "Available tests:"
+	@$(foreach t,$(RUN_TESTS),echo "  $t";)