atomic:support ATOMIC_VAR_INIT in others platform

Summary:
  This change mainly appears in the compilation environment of MVSC
  Since MVSC supports the c++11 standard in VS2019 preview3 (https://en.cppreference.com/w/cpp/compiler_support/11), we can do static initialization using the c++11 approach
atomic_int a = 1

Signed-off-by: chenrun1 <chenrun1@xiaomi.com>

include/nuttx/atomic.h

@@ -80,6 +80,9 @@ extern "C++"
 #    endif
 #    include <stdbool.h>
 #    include <stdatomic.h>
+#    ifndef ATOMIC_VAR_INIT
+#      define ATOMIC_VAR_INIT(value) (value)
+#    endif
 #  else
 #    include <nuttx/lib/stdatomic.h>
 #  endif