Use more constants and functions from libc #201

Job	Run time
aarch64-apple-darwin	41s
aarch64-apple-darwin force-inprocess	41s
aarch64-apple-darwin async	46s
x86_64-apple-darwin	1m 16s
x86_64-apple-darwin force-inprocess	1m 9s
x86_64-apple-darwin async	1m 38s
x86_64-unknown-linux-gnu	1m 3s
x86_64-unknown-linux-gnu force-inprocess	1m 1s
x86_64-unknown-linux-gnu async	1m 1s
x86_64-pc-windows-msvc	1m 57s
x86_64-pc-windows-msvc force-inprocess	2m 44s
x86_64-pc-windows-msvc async	3m 28s
i686-pc-windows-msvc	2m 56s
i686-pc-windows-msvc force-inprocess	3m 35s
i686-pc-windows-msvc async	2m 53s
x86_64-unknown-linux-gnu memfd	57s
Result	1s
	27m 47s

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