[Libomptarget] Revert new variable sharing to use the old method

The new method of sharing variables introduces a `__kmpc_alloc_shared` call
that cannot be removed in the middle end because of its non-constant argument
and unconnected free. This patch reverts this to the old method that used a
static amount of shared memory for sharing variables.

Reviewed By: jdoerfert

Differential Revision: https://reviews.llvm.org/D106905