[SYCL] [NATIVECPU] Implement generic atomic store for generic target

[PietroGhg]

Implements __spirv_AtomicStore similarly to https://github.com/intel/llvm/pull/13249. Note that the IMPL macro has been extended to take in a SUB parameter, similarly to what happens for amdgcn and ptx.

[PietroGhg]

@frasercrmck I've just realized that after https://github.com/intel/llvm/pull/13109, _CLC_GENERIC_AS_SUPPORTED will actually be 0 for Native CPU, which means that we the current use of that macro the overloads will not be available on Native CPU, which kinda defies the purpose of this PR, so I am not really sure what to do here

[PietroGhg]

@intel/llvm-gatekeepers this looks ready to merge, thank you