[lli] Add a filter to avoid importing the process's main symbol.

If JIT'd code fails to define a main function and we import the process's
definition then we will end up recursively calling lli's main until we overflow
the stack and crash. This filter fixes the issue by ensuring that the process's
main function is never imported. This results in lli producing a much friendlier
"symbol not found" error when JIT'd code fails to define main.