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.
fix Issue 9245 - [CTFE] postblit not called on static array initialization

On element-wise or block assignment, currently postblit call is implicitly
handled by glue-layer. Therefore, interpreter should have its own postblit
handling.
Fix Issue 10285 - Enum member with a type but no identifier should not be allowed.
fix Issue 12554 - [ICE](struct.c line 898) with failed delegate purity
add semantics for @nogc
Merge pull request #3490 from WalterBright/more-nogc

DIP60: add semantics for @nogc
small refactor
Merge pull request #3494 from WalterBright/refactor-isNogc

small refactor
fix Issue 12630 - @nogc should recognize compile-time evaluation context
Fix Issue 11620 - Emit enum member values to json output.
Merge pull request #3472 from AndrejMitrovic/Fix11620

Issue 11620 - Emit enum member values to json output.
Merge pull request #3493 from 9rnsr/fix12630

Issue 12630 - @nogc should recognize compile-time evaluation context
Merge pull request #3492 from 9rnsr/fix12554

[REG2.066a] Issue 12554 - [ICE](struct.c line 898) with failed delegate purity
Merge pull request #3487 from AndrejMitrovic/Fix10285

Issue 10285 - Enum member with a type but no identifier should not be allowed.
Merge pull request #3488 from 9rnsr/fix9245

Issue 9245 - [CTFE] postblit not called on static array initialization
Merge pull request #3486 from MartinNowak/MachO_CDATA

Use constant data sections for MachO
fix Issue 12606 - Mismatch of known array length during dynamic => static array assignment should emit better diagnostics

Recognize slice expression on initializer and rhs of assignment
More compile-time length check on element-wise assignment