Fix testcases for PR c/78304

The testcases as written made assumptions about size_t and long
being invalid for use with "%u".

We only need some invalid type, so this patch converts them to
attempt a "const char *" with "%u", which should be invalid for
every target.

gcc/testsuite/ChangeLog:
	PR c/78304
	* gcc.dg/format/pr78304.c: Convert argument from integral type
	to a pointer.
	* gcc.dg/format/pr78304-2.c: Likewise.

From-SVN: r244502