format_pieces-selftests.c: Silence ARI warnings

format_pieces-selftests.c: Silence ARI warnings

Silence this:

unittests/format_pieces-selftests.c:51: warning: code: Do not use printf("%ll"), instead use printf("%s",phex()) to dump a `long long' value
unittests/format_pieces-selftests.c:56: warning: code: Do not use printf("%ll"), instead use printf("%s",phex()) to dump a `long long' value

gdb/ChangeLog:

	* unittests/format_pieces-selftests.c (test_format_specifier):
	Add ARI comments.