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_internal.cpp b/libc/bionic/pthread_internal.cpp
index c6426ed..4f2ad0c 100644
--- a/libc/bionic/pthread_internal.cpp
+++ b/libc/bionic/pthread_internal.cpp
@@ -259,8 +259,7 @@
   g_func = func;
   g_arg = arg;
 
-  static _Atomic(bool) g_retval;
-  atomic_init(&g_retval, true);
+  static _Atomic(bool) g_retval(true);
 
   auto handler = [](int, siginfo_t*, void*) {
     ErrnoRestorer restorer;