arm: Better document memory barriers for Unsafe/VarHandle.

Defer to the codegen for the memory barriers instead of
explicit calls to assembler to emit the DMB instruction.
This use of MemBarrierKind allows us to better express
the required ordering even though codegen then generates
the same code anyway.

Test: testrunner.py --target --32 --optimizing
Bug: 71781600
Change-Id: I80cd8a6040b9f58ade4aa2adc9f98cfdfbdc758e
1 file changed