adding SUN_LEN macro to linux glibc/musl

[devnexen]

glibc ref musl ref

[rustbot]

r? @tgross35

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[tgross35]

I'm just going to mark this blocked since I don't yet know what to do without offset_of!. I don't think we want to add a new config specific for this macro, but maybe something like a recent-msrv cargo feature is possible.

@rustbot blocked