[MPS] Do not pass linker command to a compiler (#78630)
`-weak_framework` is a linker rather than a compiler option and as such
it should not be passed as CXX flag
Also, use `string(APPEND` rather than `set(FOO "$(FOO) ...)`
Likely fixes our ability to use `sccache` for MacOS CI builds, see https://github.com/pytorch/pytorch/issues/78375#issuecomment-1143697183
Pull Request resolved: https://github.com/pytorch/pytorch/pull/78630
Approved by: https://github.com/albanD
