Rollup merge of #103193 - krasimirgg:sysonce, r=Amanieu

mark sys_common::once::generic::Once::new const-stable

Attempt to address https://github.com/rust-lang/rust/issues/103191 by marking the impl const-stable.
Picked the declaration from the callsite:
https://github.com/rust-lang/rust/blob/21b246587c2687935bd6004ffa5dcc4f4dd6600d/library/std/src/sync/once.rs#L67

This is similar to https://github.com/rust-lang/rust/pull/98457.

With this in, `python3 x.py build library/std --target x86_64-unknown-none` succeeds.