[libc] Fix a couple issues in <wchar.h> header (#164666)
* Add FILE type declaration, as it should be presented in `<wchar.h>`,
as well as in `<stdio.h>`
* Fix argument type in `wcsrtombs` / `wcsnrtombs` function - it should
be restrict pointer to `mbstate_t`. Add restrict qualifier to internal
implementation as well.
This brings us closer to being able to build libcxx with wide-character
support against llvm-libc headers.
