rename VSXEI<EEW> to VSOXEI<EEW>

[David-Horner]

VSUXEI explicitly includes Unordered in its name to stipulate that the net result of the operation allows multiple outcomes visible to the local hart; essentially depending upon which element is last written to overlapping memory locations.

Explicitly include Ordered in the complimentary VSOXEI to stipulate that the data written to memory will be as if the active elements were first copied to an XLEN length buffer at the calculated offset processing the elements in the order from 0 to vl, and then only the affected bytes written to memory in whatever order RVVWMO allows.

The behaviour of VSUXEI should also be similarly stipulated: that the data written to memory will be as if the active elements were first copied to an XLEN length buffer at the calculated offset, processing each elements in any order, and then only the affected bytes written to memory in whatever order RVVWMO allows.e

The ideas are that

1. rather than suggesting VSUXEI is the exception to the rule, it indicates that the ordered VSOXEI is a more constrained alternative to the default processor model that allows concurrent execution wherever possible and
2. VSOXEI makes no guarantee on what is globally visible than what the RVVWMO model allows
3. VSUXEI is still constrained by the RVVWMO rules.