perf: nameToCLabel: Directly manipulate ByteString rather than going via strings

`nameToCLabel` is called from `lookupHsSymbol` many times during
bytecode linking. We can save a lot of allocations and time by directly
manipulating the bytestrings rather than going via intermediate lists.

Before: 2GB allocation, 1.11s
After: 260MB allocation, 375ms

Fixes #25719

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Metric Decrease:
    MultiLayerModulesTH_OneShot
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