Swift: repair RPATH handling for macOS
The configuration previously handled Linux properly but did not function
on macOS as ld64
does not support :
delimited paths. Account for
that by setting it to the empty string which will use multiple
invocations of the -Xlinker -rpath -Xlinker ...
pattern to compute the
correct RPATH.
Backport: release
Topic-rename: swift-rpath
Edited by Brad King