libc++: bring back the unsigned in the return type in wcstoull_l

got remove here:
https://github.com/llvm/llvm-project/commit/67b0b02ec9f2bbc57bf8f0550828d97f460ac11f#diff-e41832b8aa26da45585a57c5111531f2e1d07e91a67c4f8bf1cd6d566ae45a2bR42

Differential Revision: https://reviews.llvm.org/D141208

(cherry picked from commit fc874529)