[ORC] Use uint8_t rather than char for RPC wrapper-function calls.

[ORC] Use uint8_t rather than char for RPC wrapper-function calls.

This partially reverts 838490de7ed, which broke some Solaris bots. Apparently
Solaris defines int8_t as char rather than signed char, which made the
SerializationTypeName<char> specialization a redefinition.

This partial revert isolates use of uint8_t buffers to ORC-RPC handling of
wrapper functions only. The TargetProcessControl::runWrapper method will
continue to use char buffers.