Redo: Avoid stub when calling to write-barrier from C2, remove all related code

[Aleksey Shipilev]

On 11/20/18 7:14 PM, Roman Kennke wrote:
> This is a more conservative version of the previous patch. It preserves
> the exact code paths for interpreter and C1 versions of the stub,
> including saving+restoring the vector regs. C2 is changed to call
> straight into runtime as C2 obeys the C calling convention (albeit a
> little conservatively, apparently disabling XMM spills around calls).
> 
> http://cr.openjdk.java.net/~rkennke/remove-c2-wb-stub/webrev.03/

OK, let's try to push this one.

-Aleksey