[testsuite] Require alloca support in a few tests

Require effective target alloca in a few test-cases.

Tested on nvptx.

gcc/testsuite/ChangeLog:

2022-02-10  Tom de Vries  <tdevries@suse.de>

	* c-c++-common/Walloca-larger-than.c: Require effective target alloca.
	* c-c++-common/Warray-bounds-9.c: Same.
	* c-c++-common/Wdangling-pointer-2.c: Same.
	* c-c++-common/Wdangling-pointer-4.c: Same.
	* c-c++-common/Wdangling-pointer-5.c: Same.
	* c-c++-common/Wdangling-pointer.c: Same.
	* c-c++-common/auto-init-11.c: Same.
	* c-c++-common/auto-init-12.c: Same.
	* c-c++-common/auto-init-15.c: Same.
	* c-c++-common/auto-init-16.c: Same.
	* c-c++-common/torture/builtin-clear-padding-4.c: Same.
	* gcc.c-torture/compile/pr99787-1.c: Same.
	* gcc.dg/Walloca-larger-than-4.c: Same.
	* gcc.dg/Wdangling-pointer.c: Same.
	* gcc.dg/Wfree-nonheap-object-2.c: Same.
	* gcc.dg/Wfree-nonheap-object.c: Same.
	* gcc.dg/Wstringop-overflow-56.c: Same.
	* gcc.dg/Wstringop-overflow-57.c: Same.
	* gcc.dg/Wstringop-overflow-67.c: Same.
	* gcc.dg/Wstringop-overflow-71.c: Same.
	* gcc.dg/Wvla-larger-than-5.c: Same.
	* gcc.dg/analyzer/taint-alloc-1.c: Same.
	* gcc.dg/analyzer/torture/ubsan-1.c: Same.
	* gcc.dg/graphite/pr99085.c: Same.
	* gcc.dg/pr100225.c: Same.
	* gcc.dg/pr98721-1.c: Same.
	* gcc.dg/pr99122-2.c: Same.
	* gcc.dg/sso-14.c: Same.
	* gcc.dg/tree-ssa/builtin-sprintf-warn-25.c: Same.
	* gcc.dg/uninit-38.c: Same.
	* gcc.dg/uninit-39.c: Same.
	* gcc.dg/uninit-41.c: Same.
	* gcc.dg/uninit-pr100250.c: Same.
	* gcc.dg/uninit-pr101300.c: Same.
	* gcc.dg/uninit-pr101494.c: Same.
	* gcc.dg/uninit-pr98578.c: Same.
	* gcc.dg/uninit-pr98583.c: Same.
	* gcc.dg/vla-stexp-1.c: Same.
	* gcc.dg/vla-stexp-2.c: Same.
	* gcc.dg/vla-stexp-4.c: Same.
	* gcc.dg/vla-stexp-5.c: Same.