efi/printf: Add support for wchar_t (UTF-16)

Support %lc and %ls to output UTF-16 strings (converted to UTF-8).

Signed-off-by: Arvind Sankar <nivedita@alum.mit.edu>
Link: https://lore.kernel.org/r/20200518190716.751506-21-nivedita@alum.mit.edu


Signed-off-by: Ard Biesheuvel <ardb@kernel.org>