[libc] Support `%lc` in printf

[michaelrj-google]

Currently our printf implementation just ignores the length modifier for char. We should use the internal wcrtomb to first convert the wchar to utf-8 then write it to the output.

Standard: https://www.open-std.org/jtc1/sc22/wg14/www/docs/n3220.pdf#page=343

[llvmbot]

@llvm/issue-subscribers-libc

Author: Michael Jones (michaelrj-google)

Currently our printf implementation just ignores the length modifier for char. We should use the internal `wcrtomb` to first convert the wchar to utf-8 then write it to the output.

Standard: https://www.open-std.org/jtc1/sc22/wg14/www/docs/n3220.pdf#page=343

[shubhe25p]

I can work on it. Please assign it to me