gnustl: Fix if/elif/else library choosing
gnustl: Fix if/elif/else library choosing

We'd end up picking arm on x86 :)
Add an assertion to avoid such future mistakes

justinjoy/cerbero,centricular/cerbero,freedesktop-unofficial-mirror/gstreamer__sdk__cerbero,ramaxlo/cerbero,nzjrs/cerbero,shoreflyer/cerbero,lubosz/cerbero,lubosz/cerbero,jackjansen/cerbero,flexVDI/cerbero,shoreflyer/cerbero,ikonst/cerbero,GStreamer/cerbero,ikonst/cerbero,nzjrs/cerbero,atsushieno/cerbero,EricssonResearch/cerbero,nirbheek/cerbero,multipath-rtp/cerbero,AlertMe/cerbero,atsushieno/cerbero,jackjansen/cerbero,OptoFidelity/cerbero,shoreflyer/cerbero,flexVDI/cerbero,jackjansen/cerbero,sdroege/cerbero,freedesktop-unofficial-mirror/gstreamer__sdk__cerbero,fluendo/cerbero,ford-prefect/cerbero,justinjoy/cerbero,ramaxlo/cerbero,EricssonResearch/cerbero,nirbheek/cerbero-old,multipath-rtp/cerbero,ford-prefect/cerbero,freedesktop-unofficial-mirror/gstreamer__cerbero,nirbheek/cerbero,sdroege/cerbero,centricular/cerbero,multipath-rtp/cerbero,AlertMe/cerbero,sdroege/cerbero,multipath-rtp/cerbero,jackjansen/cerbero,freedesktop-unofficial-mirror/gstreamer__cerbero,lubosz/cerbero,nirbheek/cerbero-old,ikonst/cerbero,flexVDI/cerbero,OptoFidelity/cerbero,flexVDI/cerbero,AlertMe/cerbero,superdump/cerbero,nirbheek/cerbero-old,fluendo/cerbero,shoreflyer/cerbero,multipath-rtp/cerbero,EricssonResearch/cerbero,ikonst/cerbero,AlertMe/cerbero,freedesktop-unofficial-mirror/gstreamer__cerbero,nirbheek/cerbero,fluendo/cerbero,AlertMe/cerbero,GStreamer/cerbero,freedesktop-unofficial-mirror/gstreamer__cerbero,fluendo/cerbero,shoreflyer/cerbero,nzjrs/cerbero,ramaxlo/cerbero,OptoFidelity/cerbero,centricular/cerbero,EricssonResearch/cerbero,nzjrs/cerbero,atsushieno/cerbero,flexVDI/cerbero,OptoFidelity/cerbero,sdroege/cerbero,ikonst/cerbero,nzjrs/cerbero,fluendo/cerbero,ramaxlo/cerbero,justinjoy/cerbero,sdroege/cerbero,justinjoy/cerbero,GStreamer/cerbero,centricular/cerbero,freedesktop-unofficial-mirror/gstreamer__sdk__cerbero,superdump/cerbero,nirbheek/cerbero,centricular/cerbero,ford-prefect/cerbero,atsushieno/cerbero,nirbheek/cerbero-old,GStreamer/cerbero,superdump/cerbero,lubosz/cerbero,freedesktop-unofficial-mirror/gstreamer__sdk__cerbero,EricssonResearch/cerbero,superdump/cerbero,ramaxlo/cerbero,ford-prefect/cerbero,atsushieno/cerbero,GStreamer/cerbero