do not require compilerwrapper passed explicitly to icecc-create-env

It's an internal binary, and it should always be the same one, so
just do not require the argument and fill it in automatically.
move EndMsg to the right place

It's somewhat confusing in the finishing block, when it should actually
happen only after all the FileChunkMsg's are sent.
make -gsplit-dwarf force local building, as it outputs also .dwo file
test that results for -g are identical

The .o files are not necessary identical because of the different
way the source file is passed to the compiler, so check using gdb.