[compiler-rt] Do not search for sysroots for disabled Darwin targets

[cofibrant]

This patch improves CMake configuration time for Darwin builds by removing the need to search for sysroots for disabled targets. In particular, when the disabled targets are not installed, this search takes a while to fail.

CC @fhahn @citymarina

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[citymarina]

Thanks for doing this. This LGTM but I'd prefer that some other reviewers have a look too.

[cofibrant]

Ping