[OpenMP][libomptarget] Improve kernel initialization in plugins

This patch modifies the plugins so that the initialization of KernelTy objects
is done in the init method. Part of the initialization was done in the
constructKernelEntry method. Now this method is called constructKernel
and only allocates and constructs a KernelTy object.

This patch prepares the kernel class for the new implementation of device
reductions.

Differential Revision: https://reviews.llvm.org/D156917