[OpenMP] Add support for dynamic shared memory in new RTL

This patch adds support for using dynamic shared memory in the new
device runtime. The new function `__kmpc_get_dynamic_shared` will return a
pointer to the buffer of dynamic shared memory. Currently the amount of memory
allocated is set by an environment variable.

In the future this amount will be added to the amount used for the smart stack
which will be configured in a similar way.

Reviewed By: tianshilei1992

Differential Revision: https://reviews.llvm.org/D110006