aarch64: Register clash in debug build

[Doerr, Martin]

Hi,

we had run -Xcomp tests on aarch64 and got SIGSEGV in StubRoutines::generic_arraycopy (debug build).

generate_generic_copy:
rscratch2_dst_klass = rscratch2 = r9

It uses
MacroAssembler::check_klass_subtype_slow_path
which overwrites rscratch2 in debug build.

We don't support aarch64 at the moment, but I guess somebody is interesting in fixing it.

Best regards,
Martin