RFR: Fix aarch64

[Roman Kennke]

I tried to run some applications with Shenandoah on aarch64 and ran into
an assert in the interpreter. Apparently we got some overlap of
registers in the interpreter barriers. Changing rscratch2 to rscratch1
(the same registers that are used in the corresponding G1 barriers) then
it all works.

Tests: hotspot_gc_shenandoah (modulo 1 test that does timeout for other
reasons)

http://cr.openjdk.java.net/~rkennke/fixaarch64/webrev.00/

Ok?

Roman