Support running Spinel as a benchmark target

[k0kubun]

This PR adds support to run spinel on ruby-bench.

git clone https://github.com/matz/spinel
cd spinel
make deps
make
make install PREFIX=/opt/rubies/spinel

$ ./run_benchmarks.rb str_concat --chruby 'zjit::after --zjit;spinel' --warmup=10 --bench=10 --no-sudo
Running benchmark "str_concat" (1/1)
+ setarch x86_64 -R taskset -c 6 /opt/rubies/after/bin/ruby --zjit -I harness /home/k0kubun/src/github.com/ruby/ruby-bench/benchmarks/str_concat.rb
ruby 4.1.0dev (2026-10-02T21:46:24Z master 5b6f593531) +ZJIT +PRISM [x86_64-linux]
itr:   time
 #1:   26ms
 #2:   20ms
 #3:   22ms
 #4:   17ms
 #5:   17ms
 #6:   22ms
 #7:   21ms
 #8:   16ms
 #9:   16ms
#10:   17ms
#11:   18ms
#12:   17ms
#13:   17ms
#14:   16ms
#15:   21ms
#16:   17ms
#17:   18ms
#18:   22ms
#19:   21ms
#20:   19ms
ZJIT stats:
code_region_bytes:          8,192
zjit_alloc_bytes:          20,500
compile_time               1.10ms
profile_time               0.02ms
gc_time                    0.02ms
invalidation_time          0.00ms
RSS: 31.6MiB
MAXRSS: 95.2MiB
RSS sampled (n=10): median 80.1MiB ± 15.2MiB (MAD), range [41.5, 95.3]MiB
Average of last 10, non-warmup iters: 19ms
Running benchmark "str_concat" (1/1)
+ setarch x86_64 -R taskset -c 6 /opt/rubies/spinel/bin/spinel -E -I harness /home/k0kubun/src/github.com/ruby/ruby-bench/benchmarks/str_concat.rb
spinel 4.0.7 (2026.09.12+4635 revision 3523c3655) [x86_64-linux]
itr:   time
 #1:   10ms
 #2:    8ms
 #3:    8ms
 #4:    8ms
 #5:    8ms
 #6:    9ms
 #7:    9ms
 #8:    8ms
 #9:    8ms
#10:    8ms
#11:    8ms
#12:   10ms
#13:    8ms
#14:    8ms
#15:    8ms
#16:    8ms
#17:    9ms
#18:    8ms
#19:    8ms
#20:   10ms
RSS: 18.2MiB
Average of last 10, non-warmup iters: 8ms
Total time spent benchmarking: 2s

zjit: ruby 4.1.0dev (2026-10-02T21:46:24Z master 5b6f593531) +ZJIT +PRISM [x86_64-linux]
spinel: spinel 2026.09.12+4635 (3523c3655) [gcc 13.3.0 (cc)]

----------  ------------  -----------  --------------  -----------
bench          zjit (ms)  spinel (ms)  spinel 1st itr  zjit/spinel
str_concat  19.0 ± 10.7%   8.9 ± 9.6%           2.514        2.125
----------  ------------  -----------  --------------  -----------

[eregon]

This is redundant with the comment in harness-spinel/harness.rb so it seems best to drop it.
(the usual issue with LLM repeating things many times in many places)

[eregon]

Nice