[lli] Improve support for MinGW by implementing __main as a no-op.

Without this function lli will error out on MinGW with a missing symbol error
for __main.

Cygwin and MinGW insert calls from the main function to the runtime function
__main. The __main function is responsible for setting up main's environment
(e.g. running static constructors), however this is not needed when running
under lli: the executor process will have run non-JIT ctors, and ORC will take
care of running JIT'd ctors. To avoid a missing symbol error we just implement
__main as a no-op.