use const data segments for Mach-O

- add __DATA,__const section as CDATAREL for
  const data with relocations
use CDATA for strings on x86_64

- not sure why it was disabled

- also remove similar old code in elfobj.c
fix for code->cdata relocation

- The codegen assumes an indirect access to data,
  but cdata was treated as code segment so a direct
  relocation was emitted.
Merge pull request #3486 from MartinNowak/MachO_CDATA

Use constant data sections for MachO