[libomptarget][cuda] Call v2 functions explicitly

[libomptarget][cuda] Call v2 functions explicitly

rtl.cpp calls functions like cuMemFree that are replaced by a macro
in cuda.h with cuMemFree_v2. This patch changes the source to use
the v2 names consistently.

See also D95104, D95155 for the idea. Alternatives are to use a mixture,
e.g. call the macro names and explictly dlopen the _v2 names, or to keep
the current status where the symbols are replaced by macros in both files

Reviewed By: jdoerfert

Differential Revision: https://reviews.llvm.org/D95274