libstdc++: Remove dg-options "-std=gnu++2a" from XFAIL std::span tests

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.

We can also combine the { dg-require-effective-target c++2a } directive
with the dg-do selector.

We need to add the no_pch options for tests that define
_GLIBCXX_ASSERTIONS in the test, otherwise the PCH is included without
that defined.

libstdc++-v3/ChangeLog:

	* testsuite/23_containers/span/back_assert_neg.cc: Remove
	dg-options and add effective target selector to dg-do. Add
	no_pch.
	* testsuite/23_containers/span/back_neg.cc: Likewise.
	* testsuite/23_containers/span/cons_1_assert_neg.cc: Likewise.
	* testsuite/23_containers/span/cons_2_assert_neg.cc: Likewise.
	* testsuite/23_containers/span/first_2_assert_neg.cc: Likewise.
	* testsuite/23_containers/span/first_assert_neg.cc: Likewise.
	* testsuite/23_containers/span/first_neg.cc: Likewise.
	* testsuite/23_containers/span/front_assert_neg.cc: Likewise.
	* testsuite/23_containers/span/front_neg.cc: Likewise.
	* testsuite/23_containers/span/index_op_assert_neg.cc: Likewise.
	* testsuite/23_containers/span/index_op_neg.cc: Likewise.
	* testsuite/23_containers/span/last_2_assert_neg.cc: Likewise.
	* testsuite/23_containers/span/last_assert_neg.cc: Likewise.
	* testsuite/23_containers/span/last_neg.cc: Likewise.
	* testsuite/23_containers/span/subspan_2_assert_neg.cc:
	Likewise.
	* testsuite/23_containers/span/subspan_3_assert_neg.cc:
	Likewise.
	* testsuite/23_containers/span/subspan_4_assert_neg.cc:
	Likewise.
	* testsuite/23_containers/span/subspan_5_assert_neg.cc:
	Likewise.
	* testsuite/23_containers/span/subspan_6_assert_neg.cc:
	Likewise.
	* testsuite/23_containers/span/subspan_assert_neg.cc: Likewise.
	* testsuite/23_containers/span/subspan_neg.cc: Likewise.
	* testsuite/24_iterators/range_operations/advance_debug_neg.cc:
	Likewise.