[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.