genprimopcode: Don't assume the current locale is *.UTF-8, set the encoding explicitly

primops.txt contains Unicode characters:

% LC_ALL=C ./genprimopcode --data-decl < ./primops.txt
genprimopcode: <stdin>: hGetContents: invalid argument (cannot decode byte sequence starting from 226)

Hadrian must also avoid using readFile' to read primops.txt because it tries to decode the file with a locale-specific encoding.

---

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