aux/indices: use stdint.h types

aux/indices: use stdint.h types

These generated sources uses older, less portable types such as ubyte,
ushort and uint. But we have stdint.h everywhere now, so let's use those
types instead.

To stay consistent, let's talk about UINT8 etc instead of UBYTE for the
entirety of the u_indices infrastructure.

Reviewed-by: Yonggang Luo <luoyonggang@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/23853>