[CMake] Support externalizing debug info on non-Darwin platforms
[CMake] Support externalizing debug info on non-Darwin platforms

On other platorms, we use objcopy to export the debug info.

Differential Revision: https://reviews.llvm.org/D28575

git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@295481 91177308-0d34-0410-b5e6-96231b3b80d8

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