[OpenMP][DeviceRTL] Fix an issue that thread array might be corrupted

The shared memory stack in the device runtime assumes no intervined uses.
D135037 breaks the assumption, potentially causing the shared stack corruption.
This patch moves the thread array to heap memory. Since it is already the slow
path, it doesn't matter that much anyway.

Reviewed By: jhuber6

Differential Revision: https://reviews.llvm.org/D135391