remove DOCS from default target and druntime.zip

- These are the raw b&w ddoc html files that nobody uses.

- The distributed druntime docs are generated
  by dlang.org (through the doc target).
move -version=CoreDdoc to DDOCFLAGS

- DOCFMT is typically overriden when calling
  the makefile.
fix Issue 12121 - atomicLoad!(MemoryOrder.acq) doesn't require load barrier

[Issue 12121 – atomicLoad!(MemoryOrder.acq) should not emit additional code on X86](https://d.puremagic.com/issues/show_bug.cgi?id=12121)
Merge pull request #667 from MartinNowak/rmDOCS

[install] remove DOCS from default target and druntime.zip
Merge pull request #723 from MartinNowak/fix12121

fix Issue 12121 - atomicLoad!(MemoryOrder.acq) doesn't require load barrier