Use correct pointer size in join()
Use correct pointer size in join()

Implementation of the join() builtin used 8-byte bpf_probe_read() calls to load
char *'s, which doesn't work on 32-bit architectures. Use the correct pointer
size and simplify by removing the first/second temporaries in favor of advancing
expr_ directly to the next pointer.

iovisor/bpftrace,iovisor/bpftrace,iovisor/bpftrace,iovisor/bpftrace