vsprintf.c: add UTF-16 string (%ls) support

This is convenient for efi_loader which deals a lot with UTF-16.  Only
enabled with CC_SHORT_WCHAR, leaving room to add a UTF-32 version when
CC_SHORT_WCHAR is not enabled.

Signed-off-by: Rob Clark <robdclark@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Simon Glass <sjg@chromium.org>