Swift: support `-rpath` on Darwin

Darwin also has the concept of RPATH.  Additionally, the flag is
identical to that on other Unixish platforms.  Simply avoid the `-rpath`
handling on Windows.

This enables the use of `BUILD_WITH_INSTALL_RPATH` and `INSTALL_RPATH`
with Swift targets on Darwin.
7 jobs for rpath-darwin
Status Name Job ID Coverage
  External
passed ghostflow-check-master

passed ghostflow-check-release

passed ghostflow-stager

passed ghostflow-check-master

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