[regression] Linker warnings when building Miri on macOS

[RalfJung]

Since recently, Miri shows linker warnings when when building on macOS:

  warning: linker stderr: ''+zcm' is not a recognized feature for this target (ignoring feature)
           '+zcz' is not a recognized feature for this target (ignoring feature)
           +zcm' is not a recognized feature for this target (ignoring feature)
           '+zcz' is not a recognized feature for this target (ignoring feature)
           '+zcm' is not a recognized feature for this target (ignoring feature)
           '+zcz' is not a recognized feature for this target (ignoring feature)
           '+zcm' is not a recognized feature for this target (ignoring feature)
           '+zcz' is not a recognized feature for this target (ignoring feature)
           '+zcm' is not a recognized feature for this target (ignoring feature)
           '+zcz' is not a recognized feature for this target (ignoring feature)
...

Miri regression range: rust-lang/miri@3807a61...4fbd746
Rustc regression range: 73dc916...7218ebe