Auto merge of #51144 - tmccombs:unix-epoch-stable, r=Mark-Simulacrum
Auto merge of #51144 - tmccombs:unix-epoch-stable, r=Mark-Simulacrum

Stabilize SystemTime::UNIX_EPOCH

Fixes #49502

GBGamer/rust,aidancully/rust,graydon/rust,GBGamer/rust,GBGamer/rust,graydon/rust,aidancully/rust,aidancully/rust,graydon/rust,GBGamer/rust,aidancully/rust,aidancully/rust,GBGamer/rust,graydon/rust,graydon/rust,aidancully/rust,GBGamer/rust,graydon/rust,GBGamer/rust,GBGamer/rust