[OpenMP] Disable building `libomptarget` on 32-bit systems

There are plenty of assumptions in `libomptarget` and the device runtime
about the pointer size or `size_t`, etc. 32-bit systems are not supported. There
is no point to refine whole things to make it portable. This patch simply disables
building on 32-bit systems.

Fix #60121.

Reviewed By: jdoerfert

Differential Revision: https://reviews.llvm.org/D142023