[test] Invert stdlib_dir conditional sense. #33065
                
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In #32903, a substitution pseudovariable was introduced to refer to the
path containing Swift.swiftmodule. Since builds on Linux put this file
in a path with a subdirectory named after the architecture name -- and
presumably builds on mac OS do not, a conditional was required to
distinguish these two cases.
However, builds on OpenBSD for example also put Swift.swiftmodule in a
similar subdirectory like Linux. Thus, invert the sense of this
conditional: if mac OS, refer to the path without architecture name,
otherwise in all other cases (and therefore all other platforms), refer
to the architecture name subdirectory.