RFR: Backport PLAB stuff to shenandoah/jdk10

[Aleksey Shipilev]

On 05/31/2018 03:11 PM, Roman Kennke wrote:
> http://cr.openjdk.java.net/~rkennke/backport-plabs/webrev.01/

*) Please fold all three changesets into one "[backport] Use PLAB for evacuations instead of TLAB",
this would simplify the further backports

*) In thread.hpp, there is forward decl for PLAB:

  94 #ifdef INCLUDE_ALL_GCS
  95 class PLAB;
  96 #endif

...but the usages are not guarded by INCLUDE_ALL_GCS:

 328   PLAB*                  _gclab;                // Thread-local allocation buffer for GC (e.g.
evacuation)

 ...

 517   PLAB* gclab()                                  { return _gclab; }
 518   void set_gclab(PLAB* gclab)                    { _gclab = gclab; }

Otherwise looks good, thanks!

-Aleksey