[Libomptarget] Add basic support for dynamic shared memory on AMDGPU

This patchs adds the arguments necessary to allocate the size of the
dynamic shared memory via the `LIBOMPTARGET_SHARED_MEMORY_SIZE`
environment variable. This patch only allocates the memory, AMDGPU has a
limitation that shared memory can only be accessed from the kernel
directly. So this will currently only work with optimizations to inline
the accessor function.

Reviewed By: JonChesterfield

Differential Revision: https://reviews.llvm.org/D125252