std/sys/unix/time: make it easier for LLVM to optimize `Instant` subtraction.
std/sys/unix/time: make it easier for LLVM to optimize `Instant` subtraction.

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