[libcxx] Use aligned_alloc for testing instead of posix_memalign

Summary:
The `aligned_alloc` function is the C11 replacement for
`posix_memalign`. We should favor the C standard over the POSIX standard
so more C library implementations can run the tests.

Author: jhuber6

libcxx/test/libcxx/language.support/support.dynamic/new_faligned_allocation.pass.cpp

@@ -76,7 +76,7 @@ int main(int, char**) {
   test_allocations(64, 64);
   // Size being a multiple of alignment also needs to be supported.
   test_allocations(64, 32);
-  // When aligned allocation is implemented using posix_memalign,
+  // When aligned allocation is implemented using aligned_alloc,
   // that function requires a minimum alignment of sizeof(void*).
   // Check that we can also create overaligned allocations with
   // an alignment argument less than sizeof(void*).

libcxx/test/support/count_new.h

@@ -460,6 +460,9 @@ inline void* allocate_aligned_impl(std::size_t size, std::align_val_t align) {
   void* ret                   = nullptr;
 #    ifdef USE_ALIGNED_ALLOC
   ret = _aligned_malloc(size, alignment);
+#    elif TEST_STD_VER >= 17
+  size_t rounded_size = (size + alignment - 1) & ~(alignment - 1);
+  ret                 = aligned_alloc(alignment, size > rounded_size ? size : rounded_size);
 #    else
   assert(posix_memalign(&ret, std::max(alignment, sizeof(void*)), size) != EINVAL);
 #    endif