`VSO_0744055_atomic_load_8_bytes_readonly` test can be improved

[AlexGuteniev]

Two unrelated improvements possible:

1. Test the same thing for 128-bit atomic on x64. it was also initially implemented with cx16-based load, but now with SSE2, so the same issue applies not yet, actually, see <atomic>: Make 128-bit atomic_ref<T> using plain loads and stores #4480
2. It has UB and that is commented in the test, but with atmoic_ref it can be avoided

Not sure if either are worth doing. It is an old test for customer's situation, so may be preserved as it is.

[StephanTLavavej]

I have a pending overhaul to this test, although I haven't done either of these things (yet).

[AlexGuteniev]

unrelated improvements

Actually, 128-bit atomic is only activated for atomic_ref so 1 depends on 2.

[AlexGuteniev]

I also observe that using MapViewOfFileEx instead of the usual MapViewOfFile is unnecessary verbosity.