os: freeze signals constant #1767

Job	Run time
Build slim tarball	1m 37s
aarch64-darwin: with shared libraries	26m 35s
x86_64-linux: with shared libraries	29m 17s
x86_64-darwin: with shared libraries	50m 17s
aarch64-linux: with shared libraries	24m 25s
	2h 12m 11s

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