test/encoding: probe `setarch -R` and drop the arch argument

readable.sh wraps ceph-dencoder with `setarch $(uname -m) -R` to disable
ASLR on ASan builds, but the arch-qualified form also sets the personality
to that arch, which fails where setarch can't (e.g. riscv64). Use bare
`setarch -R` to only clear ASLR, and probe it first so the script falls
back to running ceph-dencoder unwrapped.

Signed-off-by: Sun Yuechi <sunyuechi@iscas.ac.cn>