Avoid warning for memset writing over multiple members.

Avoid warning for memset writing over multiple members.

Resolves:
PR middle-end/95667 - unintended warning for memset writing across multiple members
PR middle-end/92814 - missing -Wstringop-overflow writing into a dynamically allocated flexible array member

gcc/ChangeLog:

	PR middle-end/95667
	PR middle-end/92814
	* builtins.c (compute_objsize): Remove call to
	compute_builtin_object_size and instead compute conservative sizes
	directly here.

gcc/testsuite/ChangeLog:

	PR middle-end/95667
	PR middle-end/92814
	* gcc.dg/Wstringop-overflow-25.c: Remove xfails.
	* gcc.dg/Wstringop-overflow-39.c: New test.