swift234

[cmake] Replace a bunch of OSX checks with Darwin checks

These should all apply to any Darwin platform, and the current behaviour
was breaking cross-compilation.

Introduces a SWIFT_DARWIN_VARIANTS pattern, to be used as follows:
    if(SWIFT_HOST_VARIANT MATCHES "${SWIFT_DARWIN_VARIANTS}")

Also fix one place where I checked the CMAKE_SYSTEM_NAME instead of the
host variant that I recently introduced.  I haven't attempted to find
the rest of the places we're doing this though.

Swift SVN r26554