Implement .ctor support for PEi386.

Signed-off-by: Edward Z. Yang <ezyang@mit.edu>
Implement __mod_init_func for Mach-O. Finishes support for init in #5435.

Signed-off-by: Edward Z. Yang <ezyang@mit.edu>
Distinguish between hs-main cases when giving rtsopts advice.

Signed-off-by: Edward Z. Yang <ezyang@mit.edu>