[PATCH] libstdc++: Add C++2a synchronization support

[David Edelsohn]

I am seeing 93 new libstdc++ failures on AIX, even after Jonathan's
fixes.  And a few c++ failures with similar symptoms.  I'm not certain
that it is due to this patch, but it's the likely suspect.

FAIL: 17_intro/headers/c++2020/all_attributes.cc (test for excess errors)
FAIL: 17_intro/headers/c++2020/all_no_exceptions.cc (test for excess errors)
FAIL: 17_intro/headers/c++2020/all_no_rtti.cc (test for excess errors)
FAIL: 17_intro/headers/c++2020/all_pedantic_errors.cc (test for excess errors)
FAIL: 17_intro/headers/c++2020/operator_names.cc (test for excess errors)
FAIL: 17_intro/headers/c++2020/stdc++.cc (test for excess errors)
FAIL: 17_intro/headers/c++2020/stdc++_multiple_inclusion.cc (test for
excess errors)
FAIL: 20_util/allocator/rebind_c++20.cc (test for excess errors)
FAIL: 20_util/allocator/requirements/constexpr.cc (test for excess errors)
FAIL: 20_util/allocator/requirements/typedefs_c++20.cc (test for excess errors)
FAIL: 20_util/allocator_traits/header.cc (test for excess errors)
FAIL: 20_util/allocator_traits/members/92878_92947.cc (test for excess errors)
UNRESOLVED: 20_util/allocator_traits/members/92878_92947.cc
compilation failed to produce executable
FAIL: 20_util/assume_aligned/1.cc (test for excess errors)
UNRESOLVED: 20_util/assume_aligned/1.cc compilation failed to produce executable
FAIL: 20_util/assume_aligned/2_neg.cc (test for excess errors)
FAIL: 20_util/assume_aligned/3.cc (test for excess errors)
UNRESOLVED: 20_util/assume_aligned/3.cc scan-assembler-not undefined
FAIL: 20_util/assume_aligned/97132.cc (test for excess errors)
FAIL: 20_util/function_objects/bind_front/2.cc (test for excess errors)
UNRESOLVED: 20_util/function_objects/bind_front/2.cc compilation
failed to produce executable
FAIL: 20_util/pair/comparison_operators/constexpr_c++20.cc (test for
excess errors)
FAIL: 20_util/pair/cons/92878_92947.cc (test for excess errors)
UNRESOLVED: 20_util/pair/cons/92878_92947.cc compilation failed to
produce executable
etc.

The errors all are of the form:

/tmp/GCC/powerpc-ibm-aix7.2.3.0/libstdc++-v3/include/bits/atomic_wait.h:125:
error: 'mutex' is not a member of 'std'
/tmp/GCC/powerpc-ibm-aix7.2.3.0/libstdc++-v3/include/bits/atomic_wait.h:125:
error: template argument 1 is invalid
/tmp/GCC/powerpc-ibm-aix7.2.3.0/libstdc++-v3/include/bits/atomic_wait.h:125:
error: '<expression error>' in namespace 'std' does not name a type
/tmp/GCC/powerpc-ibm-aix7.2.3.0/libstdc++-v3/include/bits/atomic_wait.h:126:
error: '__lock_t' does not name a type; did you mean 'clock_t'?
/tmp/GCC/powerpc-ibm-aix7.2.3.0/libstdc++-v3/include/bits/atomic_wait.h:132:
error: '__gthread_cond_t' does not name a type; did you mean
'pthread_cond_t'?
/tmp/GCC/powerpc-ibm-aix7.2.3.0/libstdc++-v3/include/bits/atomic_wait.h:135:
error: '_M_cond' was not declared in this scope
/tmp/GCC/powerpc-ibm-aix7.2.3.0/libstdc++-v3/include/bits/atomic_wait.h:135:
error: '__GTHREAD_COND_INIT_FUNCTION' was not declared in this scope
/tmp/GCC/powerpc-ibm-aix7.2.3.0/libstdc++-v3/include/bits/atomic_wait.h:164:
error: '__lock_t' is not a member of 'std::__detail::__waiters'
/tmp/GCC/powerpc-ibm-aix7.2.3.0/libstdc++-v3/include/bits/atomic_wait.h:165:
error: '_M_cv' was not declared in this scope
/tmp/GCC/powerpc-ibm-aix7.2.3.0/libstdc++-v3/include/bits/atomic_wait.h:165:
error: '__l' was not declared in this scope; did you mean '__lg'?
/tmp/GCC/powerpc-ibm-aix7.2.3.0/libstdc++-v3/include/bits/atomic_wait.h:165:
error: '__gthread_cond_wait' was not declared in this scope; did you
mean 'pthread_cond_t'?
/tmp/GCC/powerpc-ibm-aix7.2.3.0/libstdc++-v3/include/bits/atomic_wait.h:191:
error: '_M_cv' was not declared in this scope
/tmp/GCC/powerpc-ibm-aix7.2.3.0/libstdc++-v3/include/bits/atomic_wait.h:191:
error: '__gthread_cond_broadcast' was not declared in this scope
/tmp/GCC/powerpc-ibm-aix7.2.3.0/libstdc++v3/include/bits/atomic_timed_wait.h:144:
error: '__gthread_cond_t' was not declared in this scope; did you mean
'pthread_cond_t'?
/tmp/GCC/powerpc-ibm-aix7.2.3.0/libstdc++v3/include/bits/atomic_timed_wait.h:144:
error: '__cv' was not declared in this scope