support: Fix printf format for TEST_COMPARE_STRING

Fix the following on 32 bits targets:

support_test_compare_string.c: In function ‘support_test_compare_string’:
support_test_compare_string.c:80:37: error: format ‘%lu’ expects argument of
type ‘long unsigned int’, but argument 2 has type ‘size_t’ {aka ‘unsigned int’}
[-Werror=format=]
         printf ("  string length: %lu bytes\n", left_length);
                                   ~~^           ~~~~~~~~~~~
                                   %u
Checked on arm-linux-gnueabihf.

	* support/support_test_compare_string.c
	(support_test_compare_string): Fix printf format.