Replace hard-coded POSIX dependencies with portable code

[hsbt]

YJIT now works on mswin, so I'd like to evaluate it with ruby-bench. I plan to open two PRs. This one mainly makes code that hard-codes POSIX behavior portable, and none of it is specific to Windows.

• Join PATH entries with File::PATH_SEPARATOR instead of :
• Redirect to File::NULL instead of /dev/null
• Create output directories with FileUtils.mkdir_p instead of mkdir -p
• Run benchmark commands from an argv array instead of a shelljoin-ed string, so Shellwords escapes such as \= never reach Ruby
• Compute knucleotide workers sequentially where Process.fork is unavailable
• Read zeros from a temporary file in gvl_release_acquire when /dev/zero is missing
• Run ruby bin/rails in railsbench instead of relying on the shebang

Where these fallbacks apply, knucleotide and gvl_release_acquire measure something different from Linux, so they should be left out of comparisons with Linux. Comparing two builds on the same machine is unaffected.

After this is merged, I will open a PR with the mswin-specific changes.

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