[libc++][format] Add a CMake Unicode option.

This option is used to select between the format headers output column
width option. This option should be independent of the locale setting.
It's encouraged to default to Unicode unless the platform doesn't offer
that option.

[format.string.std]/10
```
  For the purposes of width computation, a string is assumed to be in a
  locale-independent, implementation-defined encoding. Implementations
  should use a Unicode encoding on platforms capable of displaying Unicode
```

Reviewed By: #libc, ldionne, vitaut

Differential Revision: https://reviews.llvm.org/D103379