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Replace hard-coded POSIX dependencies with portable code #2105

Job	Run time
benchmark-ractor-only (head)	26s
benchmark-default (head)	11m 25s
benchmark-default (truffleruby, --excludes=knucleotide,lobsters,ruby-lsp,shipit)	11m 52s
test (truffleruby)	1m 13s
benchmark-default (ruby)	13m 8s
benchmark-graph	43s
benchmark-run-once	1m 56s
test (head)	44s
benchmark-ractor (head)	6m 33s
test (ruby)	55s
all-tests	4s
	48m 59s