RFR: Fix PLAB alignment reserve

[Roman Kennke]

The setup of AlignmentReserve in PLAB is not quite right for Shenandoah:
when oop-header-size > MinObjAlignment then it is set to header size of
int array or Shenandoah's forced min-aligment, otherwise 0. However, if
header-size == MinObjAligment, then it's set to 0. This doesn't fit
because we need an extra word. I changed the logic to compute this to
take Shenandoah's extra word into account, and this way we also don't
need to force a min_align_reserve for Shenandoah.

Passes previously (sometimes) failing LargeObjectAligment test as well
as all hotspot_gc_shenandoah tests in fastdebug.

http://cr.openjdk.java.net/~rkennke/fixplabalign/webrev.01/

Ok?