[libstdc++] define zoneinfo_dir_override on vxworks

[libstdc++] define zoneinfo_dir_override on vxworks

VxWorks fails to load kernel-mode modules with weak undefined symbols.
In RTP mode modules, that undergo final linking, weak undefined
symbols are not a problem.

This patch adds kernel-mode VxWorks multilibs to the set of targets
that don't support weak undefined symbols without special flags, in
which tzdb's zoneinfo_dir_override is given a weak definition.

for  libstdc++-v3/ChangeLog

	* src/c++20/tzdb.cc (__gnu_cxx::zoneinfo_dir_override): Define
	on VxWorks non-RTP.