[OpenMP] Add environment variables to change stack / heap size in the CUDA plugin

This patch adds support for two environment variables to configure the device.
``LIBOMPTARGET_STACK_SIZE`` sets the amount of memory in bytes that each thread
has for its stack. ``LIBOMPTARGET_HEAP_SIZE`` sets the amount of heap memory
that can be allocated using malloc / free on the device.

Reviewed By: jdoerfert

Differential Revision: https://reviews.llvm.org/D106627