RFR: Missing enter/leave around keep_alive_barrier in AArch64

[Roman Kennke]

I've been missing enter/leave calls around the SATB pre barrier call in 
MacroAssembler::keep_alive_barrier() for Shenandoah. This has been 
sending EvilSyncBug (and possible some other tests) into endless loops.

The cleanest place to have them is in the (only) user of it in 
generate_Reference_get():

http://cr.openjdk.java.net/~rkennke/aarch64-enter-leave/webrev.00/

Test: EvilSyncBug terminates now (aarch64). Running other tests right now

Ok?