Testsuite: Make dependence on -fdelete-null-pointer-checks explicit

Testsuite: Make dependence on -fdelete-null-pointer-checks explicit

nios2-elf target defaults to -fno-delete-null-pointer-checks, breaking
tests that implicitly depend on that optimization.  Add the option
explicitly on these tests.

2022-01-08  Sandra Loosemore  <sandra@codesourcery.com>

	gcc/testsuite/
	* g++.dg/cpp0x/constexpr-compare1.C: Add explicit
	-fdelete-null-pointer-checks option.
	* g++.dg/cpp0x/constexpr-compare2.C: Likewise.
	* g++.dg/cpp0x/constexpr-typeid2.C: Likewise.
	* g++.dg/cpp1y/constexpr-94716.C: Likewise.
	* g++.dg/cpp1z/constexpr-compare1.C: Likewise.
	* g++.dg/cpp1z/constexpr-if36.C: Likewise.
	* gcc.dg/init-compare-1.c: Likewise.

	libstdc++-v3/
	* testsuite/18_support/type_info/constexpr.cc: Add explicit
	-fdelete-null-pointer-checks option.