Remove #ifndef AARCH64 from drx_buf-test.dll.c

Created by: fhahn

Split from #1569

All missing XINST macros are available now, so the test builds with the ifndefs removed, but fails during execution:

~/projects/dynamorio-work/build-debug/bin64/drrun "-s" "90" "-quiet" "-debug" "-killpg" "-stderr_mask" "0xC" "-dumpcore_mask" "0" "-code_api" "-c" "~/projects/dynamorio-work/build-debug/suite/tests/bin/libclient.drx_buf-test.dll.so" "--" "~projects/dynamorio-work/build-debug/suite/tests/bin/client.drx_buf-test"
Starting drx_buf threaded test
<ERROR: using undefined symbol!>