Rollup merge of #61497 - Mark-Simulacrum:codegen-units-std-num-cpus, r=alexcrichton
Rollup merge of #61497 - Mark-Simulacrum:codegen-units-std-num-cpus, r=alexcrichton

Treat 0 as special value for codegen-units-std

Fixes #57669

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