[Libomptarget] Use scoped atomics in the device runtime (#75834)

Summary:
A recent patch allowed us to easily replace GNU atomics with scoped
variants that make use of the backend's handling for more permissive
scopes. The default is full "system" scope, that means the atomic
operation must be consistent with operations that may happen on the
host's memory. This is generally only required for processes that are
communicating with something via global fine-grained memory. This patch
uses these atomics to make everything device scoped, as nothing in the
OpenMP runtime should depend on the host.

This is only provided as a very new clang extension but the DeviceRTL is
only compiled with clang so it is always available.