bionic: libc: avoid -Wdeprecated-declarations via std::atomic_init

std::atomic_init is deprecated in C++20, and is slated for removal in C++26.
Replace the usage of std::atomic_init with std::atomic_store_explicit with
std::memory_ordering_relaxed.

Link: https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2019/p0883r2.pdf
Link: https://github.com/llvm/llvm-project/commit/56aac567acfd696f54163e33d8df02dc2ad3a72e
Test: mmma bionic
Change-Id: Idf42aea193cfacf8dd7f8528560a396c6064468c
diff --git a/libc/bionic/pthread_mutex.cpp b/libc/bionic/pthread_mutex.cpp
index 0a452e9..c99717a 100644
--- a/libc/bionic/pthread_mutex.cpp
+++ b/libc/bionic/pthread_mutex.cpp
@@ -509,8 +509,8 @@
     memset(mutex, 0, sizeof(pthread_mutex_internal_t));
 
     if (__predict_true(attr == nullptr)) {
-        atomic_init(&mutex->state, MUTEX_TYPE_BITS_NORMAL);
-        return 0;
+      atomic_store_explicit(&mutex->state, MUTEX_TYPE_BITS_NORMAL, memory_order_relaxed);
+      return 0;
     }
 
     uint16_t state = 0;
@@ -543,13 +543,13 @@
         }
         mutex->pi_mutex_id = id;
 #endif
-        atomic_init(&mutex->state, PI_MUTEX_STATE);
+        atomic_store_explicit(&mutex->state, PI_MUTEX_STATE, memory_order_relaxed);
         PIMutex& pi_mutex = mutex->ToPIMutex();
         pi_mutex.type = *attr & MUTEXATTR_TYPE_MASK;
         pi_mutex.shared = (*attr & MUTEXATTR_SHARED_MASK) != 0;
     } else {
-        atomic_init(&mutex->state, state);
-        atomic_init(&mutex->owner_tid, 0);
+      atomic_store_explicit(&mutex->state, state, memory_order_relaxed);
+      atomic_store_explicit(&mutex->owner_tid, 0, memory_order_relaxed);
     }
     return 0;
 }