[libc] Remove use of BlockStore for GPU atexit (#82823)

Summary:
The GPU backends have restrictions on the kinds of initializers they can
produce. The use of BlockStore here currently breaks the backends
through the use of recursive initializers. This prevents it from
actually being included in any builds. This patchs changes it to just
use a fixed size of 64 slots .The chances of someone exceeding the 64
slots in practice is very, very low.

However, this is primarily a bandaid solution as a real solution will
need to use a lock free data structure to push work in parallel.
Currently the mutexes on the GPU build do nothing, so they only work if
the user guards the use themselves.

Author: jhuber6

libc/src/stdlib/atexit.cpp

@@ -28,7 +28,14 @@ struct AtExitUnit {
   constexpr AtExitUnit(AtExitCallback *c, void *p) : callback(c), payload(p) {}
 };
 
-#ifdef LIBC_COPT_PUBLIC_PACKAGING
+#if defined(LIBC_TARGET_ARCH_IS_GPU)
+// The GPU build cannot handle the potentially recursive definitions required by
+// the BlockStore class. Additionally, the liklihood that someone exceeds this
+// while executing on the GPU is extremely small.
+// FIXME: It is not generally safe to use 'atexit' on the GPU because the
+//        mutexes simply passthrough. We will need a lock free stack.
+using ExitCallbackList = FixedVector<AtExitUnit, 64>;
+#elif defined(LIBC_COPT_PUBLIC_PACKAGING)
 using ExitCallbackList = cpp::ReverseOrderBlockStore<AtExitUnit, 32>;
 #else
 // BlockStore uses dynamic memory allocation. To avoid dynamic memory