Fix -split-obj on Mac OS via -fasm

Fix -split-obj on Mac OS via -fasm

The problem of the splitter was that it re-emitted section directives
for every dynamic label found. The following was torn apart

.symbol_stubs
.indirect <symbol>
L_<symbol>$stub:
	jmp *...
L_<symbol>$stub_binder:
	..somebinding code..

into

.symbol_stubs
.indirect_symbol <symbol>
L_<symbol>$stub:
	jmp *...
.symbol_stubs <--- NEW
L_<symbol>$stub_binder:
	..somebinding code..

This is incorrect as the Mac OS assembler enforces that every new code
section that goes into .symbol_stubs is associated with the linker
directive .indirect_symbol. This sanity check is obviously violated
when we reemit .symbol_stub in the splitter. The solution is to ignore
everything that ends with $stub_binder as new label, and chuck it into
a single label for $stub.

Also the splitter has to recognize .section __DATA... for the lazy_ptr
indirection symbol. Adds a reminder to PositionIndependentCode.hs to
take care of the splitter when the code generation is changed.

This should not affect -fvia-c as the code generated by the C compiler
is entirely different.