libstdc++: Replace dg-options "-std=c++20" with dg-add-options strict_std

The testsuite will automatically select C++20 for these tests now, and
removing the hardcoded -std option allows them to be tested for C++23
and C++26 as well. Because they test a problem seen with -std=c++20 add
the new { dg-add-options strict_std } directive so that the test runner
uses -std=c++NN not -std=gnu++NN.

libstdc++-v3/ChangeLog:

	* testsuite/24_iterators/reverse_iterator/100639.cc: Remove
	dg-options and add dg-add-options strict_std.
	* testsuite/std/ranges/iota/93267.cc: Likewise.
	* testsuite/std/ranges/iota/96042.cc: Likewise.
	* testsuite/std/ranges/iota/size.cc: Likewise.
	* testsuite/std/ranges/subrange/96042.cc: Likewise.