[SYCL] `ProgramManager::m_EliminatedKernelArgMasks` is not protected by a mutex

[jopperm]

In contrast to other, similar members in the ProgramManager class, accesses to the m_EliminatedKernelArgMasks map are not protected by a mutex (e.g. [1], [2]). The code documentation does not explain whether this is intentional. We should investigate whether a mutex is needed, and either update the documentation, use the existing m_BuiltInKernelIDsMutex like most of the other members, or introduce a new mutex.