[compiler-rt][Darwin] Refactor minimum deployment target substitutions

* Support macOS 11+ version scheme
* Standardize substitution name `%min_deployment_target=x.y`
* Remove unneeded error cases (the input version is hard-coded)
* Specify version as tuple instead of string; no need to parse it

These changes should also facilitate a future addition of a substitution
that expands to "set deployment target to current target version"
(https://reviews.llvm.org/D70151).

Reviewed By: delcypher

Differential Revision: https://reviews.llvm.org/D85925