Add manually overriding of the darwin SDK version

Summary:
When doing cross-compilation from Linux to MacOS we don't have
access to have access to `xcodebuild` and therefore need a way
to set the SDK version from the outside.

Fixes https://reviews.llvm.org/D68292#1853594 for me.

Reviewers: delcypher, yln

Reviewed By: delcypher

Subscribers: #julialang, mgorny, #sanitizers

Tags: #sanitizers

Differential Revision: https://reviews.llvm.org/D77026