RFR: Add null-checking to write-barriers in aarch64 C1 cas and xchg impls.

[Roman Kennke]

In AArch64's C1, we need null-checking in cas and xchg implementations,
otherwise we get crashes when running jcstress. Same as in x86.

http://cr.openjdk.java.net/~rkennke/aarch64fixlet/webrev.00/

Ok?

Roman