[libc] Use __SIZEOF_LONG__ to define LONG_WIDTH instead of sizeof(long). (#79391)

The standard requires `limits.h` constants to be used in preprocessors.
So we use `__SIZEOF_LONG__` instead of `sizeof(long)` to define
`LONG_WIDTH`. The macro `__SIZEOF_LONG__` seems to be available on both
clang and gcc since at least version 9.

Author: lntue

libc/include/llvm-libc-macros/limits-macros.h

@@ -70,8 +70,9 @@
 #elif defined(__WORDSIZE)
 #define LONG_WIDTH __WORDSIZE
 #else
-// Use sizeof(long) * CHAR_BIT as backup. This is needed for clang-13 or before.
-#define LONG_WIDTH (sizeof(long) * CHAR_BIT)
+// Use __SIZEOF_LONG__ * CHAR_BIT as backup.  This is needed for clang-13 or
+// before.
+#define LONG_WIDTH (__SIZEOF_LONG__ * CHAR_BIT)
 #endif // __LONG_WIDTH__
 #endif // LONG_WIDTH