test/encoding: use setarch -R for ASAN builds in readable.sh

test/encoding: use setarch -R for ASAN builds in readable.sh

When running readable.sh with a WITH_ASAN=ON build of ceph-dencoder,
ASAN processes need to find a contiguous 16+ TB shadow memory region
(1/8 of the 128 TB x86-64 user VA space). High ASLR entropy can
fragment the VA space, preventing ASAN from finding a suitable region.

Instead of requiring system-wide vm.mmap_rnd_bits=28 (which weakens
ASLR security for the entire host), wrap ceph-dencoder with 'setarch
$(uname -m) -R' when ASAN is detected. This disables ASLR only for the
specific ceph-dencoder processes, with no system-wide security impact.

Also simplify parallelism logic: extract NPROC calculation into a shared
variable and use it consistently across FreeBSD, Darwin, and Linux.

Reference: https://clang.llvm.org/docs/AddressSanitizer.html

Signed-off-by: Kefu Chai <k.chai@proxmox.com>