Magic symbols of the form $ld$install_name$os9.0$@rpath/libswiftCore.dylib tell the linker to use that install name when targeting that OS version. Use these symbols to specify an @rpath install name for all back-deployment libraries when targeting watchOS 2.0-5.1, iOS 7.0-12.1, and macOS 10.9-10.14.
rdar://problem/45027809
This is in preparation to make the code here more target agnostic for
porting to the Windows threading primitives. This is used pretty
extensively in the tests, so disabling tests would lose a chunk of
coverage.
