Rollup merge of #84865 - petrochenkov:llthread, r=Mark-Simulacrum
Rollup merge of #84865 - petrochenkov:llthread, r=Mark-Simulacrum

rustbuild: Pass a `threads` flag that works to windows-gnu lld

MinGW driver for COFF LLD doesn't currently translate GNU-style `--threads=N` to native `/threads:N`, so we have to pass the option in its native form to avoid an error.

Also pass the `threads` flag to lld-link (windows-msvc lld) as well.

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