Correct stpcpy offset computation for -Warray-bounds et al. [PR101397].

Correct stpcpy offset computation for -Warray-bounds et al. [PR101397].

Resolves:
PR middle-end/101397 - spurious warning writing to the result of stpcpy minus 1

gcc/ChangeLog:

	PR middle-end/101397
	* builtins.c (gimple_call_return_array): Add argument.  Correct
	offsets for memchr, mempcpy, stpcpy, and stpncpy.
	(compute_objsize_r): Adjust offset computation for argument returning
	built-ins.

gcc/testsuite/ChangeLog:

	PR middle-end/101397
	* gcc.dg/Warray-bounds-80.c: New test.
	* gcc.dg/Warray-bounds-81.c: New test.
	* gcc.dg/Warray-bounds-82.c: New test.
	* gcc.dg/Warray-bounds-83.c: New test.
	* gcc.dg/Warray-bounds-84.c: New test.
	* gcc.dg/Wstringop-overflow-46.c: Adjust expected output.