[build] Fix bootstrapping builds that disable back-deployment
This fixes a build that passes `-D SWIFT_STDLIB_SUPPORT_BACK_DEPLOYMENT=NO` and `-D BOOTSTRAPPING_MODE=BOOTSTRAPPING-WITH-HOSTLIBS`.
Previously a few CMake errors would be emitted:
```
CMake Error at cmake/modules/AddSwift.cmake:478 (get_property):
get_property could not find TARGET HostCompatibilityLibs. Perhaps it has
not yet been created.
CMake Error at cmake/modules/AddSwift.cmake:527 (add_dependencies):
add_dependencies called with incorrect number of arguments
Call Stack (most recent call first):
cmake/modules/AddSwift.cmake:969 (_add_swift_runtime_link_flags)
tools/driver/CMakeLists.txt:23 (add_swift_host_tool)
```
