[libc][NFC] Add a static assert for futex size.

libc/include/__llvm-libc-common.h

@@ -23,6 +23,9 @@
 #undef _Alignas
 #define _Alignas alignas
 
+#undef _Static_assert
+#define _Static_assert static_assert
+
 #undef _Alignof
 #define _Alignof alignof
 

libc/include/llvm-libc-types/__futex_word.h

@@ -16,6 +16,7 @@ typedef struct {
   _Alignas(sizeof(unsigned int) > _Alignof(unsigned int)
                ? sizeof(unsigned int)
                : _Alignof(unsigned int)) unsigned int __word;
+  _Static_assert(sizeof(unsigned int) == 4, "Unexpected size of unsigned int.");
 #else
 #error "A type to represent a futex word is not available for the target arch."
 #endif