sys/contrib/zlib: Always define Z_U8 and Z_U4

This is a temporary hack for zlib to make sure that the library
still builds when building with Z_SOLO (used in kernel and loader),
as zlib is depending on limits.h which is only available in STDC
case.

PR: kern/262977
MFC after: 3 days

(cherry picked from commit 0a21252adf11f7e839eabeb530e75cd1f9cd5386)

Approved by: re (gjb, early MFC)