Trust the "i128" feature

[cuviper]

If the "i128" feature is explicity requested, don't bother probing for it. It will still cause a build error if that was set improperly.

[cuviper]

bors r+

[bors[bot]]

Build succeeded:

• Format
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• Test (1.15.0)
• Test (1.20.0)
• Test (1.26.0)
• Test (1.31.0)
• Test (1.8.0)
• Test (beta)
• Test (i586)
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• Test (stable)