Support for `rb_io_mode_t`. #451

Job	Run time
ruby-versions / Generate Ruby versions	3s
ubuntu-24.04 2.7	37s
ubuntu-24.04 3.0	10s
ubuntu-24.04 3.1	11s
ubuntu-24.04 3.2	21s
ubuntu-24.04 3.3	13s
ubuntu-24.04 3.4	11s
ubuntu-24.04 debug	18s
ubuntu-24.04 head	15s
ubuntu-24.04 jruby-head	1m 0s
ubuntu-24.04 truffleruby-head	32s
ubuntu-22.04 2.7	39s
ubuntu-22.04 3.0	11s
ubuntu-22.04 3.1	15s
ubuntu-22.04 3.2	16s
ubuntu-22.04 3.3	10s
ubuntu-22.04 3.4	11s
ubuntu-22.04 debug	17s
ubuntu-22.04 head	15s
ubuntu-22.04 jruby-head	1m 6s
ubuntu-22.04 truffleruby-head	29s
	7m 40s

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