libstdc++: Remove dg-options "-std=gnu++20" from <atomic> tests

libstdc++: Remove dg-options "-std=gnu++20" from <atomic> 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.

libstdc++-v3/ChangeLog:

	* testsuite/29_atomics/atomic/compare_exchange_padding.cc:
	Remove dg-options -std=gnu++2a.
	* testsuite/29_atomics/atomic/cons/value_init.cc: Likewise.
	* testsuite/29_atomics/atomic/lock_free_aliases.cc: Likewise.
	* testsuite/29_atomics/atomic/requirements/types_neg.cc:
	Likewise.
	* testsuite/29_atomics/atomic/wait_notify/1.cc: Likewise.
	* testsuite/29_atomics/atomic/wait_notify/100334.cc: Likewise.
	* testsuite/29_atomics/atomic/wait_notify/102994.cc: Likewise.
	* testsuite/29_atomics/atomic/wait_notify/2.cc: Likewise.
	* testsuite/29_atomics/atomic/wait_notify/bool.cc: Likewise.
	* testsuite/29_atomics/atomic/wait_notify/generic.cc: Likewise.
	* testsuite/29_atomics/atomic/wait_notify/pointers.cc: Likewise.
	* testsuite/29_atomics/atomic_flag/cons/value_init.cc: Likewise.
	* testsuite/29_atomics/atomic_flag/test/explicit.cc: Likewise.
	* testsuite/29_atomics/atomic_flag/test/implicit.cc: Likewise.
	* testsuite/29_atomics/atomic_flag/wait_notify/1.cc: Likewise.
	* testsuite/29_atomics/atomic_float/1.cc: Likewise.
	* testsuite/29_atomics/atomic_float/95282.cc: Likewise.
	* testsuite/29_atomics/atomic_float/requirements.cc: Likewise.
	* testsuite/29_atomics/atomic_float/value_init.cc: Likewise.
	* testsuite/29_atomics/atomic_float/wait_notify.cc: Likewise.
	* testsuite/29_atomics/atomic_integral/cons/value_init.cc:
	Likewise.
	* testsuite/29_atomics/atomic_integral/wait_notify.cc: Likewise.
	* testsuite/29_atomics/atomic_ref/compare_exchange_padding.cc:
	Likewise.
	* testsuite/29_atomics/atomic_ref/deduction.cc: Likewise.
	* testsuite/29_atomics/atomic_ref/float.cc: Likewise.
	* testsuite/29_atomics/atomic_ref/generic.cc: Likewise.
	* testsuite/29_atomics/atomic_ref/integral.cc: Likewise.
	* testsuite/29_atomics/atomic_ref/pointer.cc: Likewise.
	* testsuite/29_atomics/atomic_ref/requirements.cc: Likewise.
	* testsuite/29_atomics/headers/atomic/types_std_c++20.cc:
	Likewise.
	* testsuite/29_atomics/headers/atomic/types_std_c++20_neg.cc:
	Likewise.
	* testsuite/29_atomics/headers/atomic/types_std_c++2a.cc:
	Likewise.
	* testsuite/29_atomics/headers/atomic/types_std_c++2a_neg.cc:
	Likewise.