[libc-commits] [libc] [libc] Fix missing UINTMAX_WIDTH (PR #87092)

[Nick Desaulniers via libc-commits]

https://github.com/nickdesaulniers approved this pull request.

Could also do something like:
```
#ifndef UINTMAX_WIDTH
#define UINTMAX_WIDTH sizeof(uintmax_t) * CHAR_BIT
#endif
```
But happy to take this to unbreak builds.

https://github.com/llvm/llvm-project/pull/87092