[llvm-exegesis] Replace size_t with ssize_t where relevant

Currently BenchmarkRunner.cpp stores the return code of recvmsg as
size_t. Not only is this incorrect (as recvmsg returns ssize_t), but it
also makes the error code check after the statement completely irrelvant
as it checks if the number of bytes read is greater than zero (which
will always be true for an unsigned type).