iosbuild.sh: Support macosx targets in Xcode 7.
iosbuild.sh: Support macosx targets in Xcode 7.

Xcode 7 refuses to link to x86 and x86_64 code that's built for
iphone sim, so add an extra command line flag that forces iosbuild
to use darwin15 targets.

Change-Id: I2228d458f5cccf4d26866040380a974f88d9d360

GrokImageCompression/aom,jmvalin/aom,mbebenita/aom,GrokImageCompression/aom,mwgoldsmith/vpx,mbebenita/aom,openpeer/libvpx_new,ittiamvpx/libvpx-1,mbebenita/aom,mbebenita/aom,webmproject/libvpx,ittiamvpx/libvpx-1,GrokImageCompression/aom,Topopiccione/libvpx,luctrudeau/aom,Topopiccione/libvpx,mwgoldsmith/vpx,openpeer/libvpx_new,mwgoldsmith/vpx,mwgoldsmith/libvpx,luctrudeau/aom,Topopiccione/libvpx,shacklettbp/aom,openpeer/libvpx_new,Topopiccione/libvpx,thdav/aom,openpeer/libvpx_new,webmproject/libvpx,mbebenita/aom,jmvalin/aom,webmproject/libvpx,Topopiccione/libvpx,luctrudeau/aom,mwgoldsmith/libvpx,ittiamvpx/libvpx-1,thdav/aom,ittiamvpx/libvpx-1,mbebenita/aom,webmproject/libvpx,mbebenita/aom,mwgoldsmith/vpx,jmvalin/aom,jmvalin/aom,openpeer/libvpx_new,smarter/aom,mbebenita/aom,jmvalin/aom,thdav/aom,shacklettbp/aom,luctrudeau/aom,ShiftMediaProject/libvpx,openpeer/libvpx_new,luctrudeau/aom,Topopiccione/libvpx,shacklettbp/aom,ShiftMediaProject/libvpx,mwgoldsmith/libvpx,ittiamvpx/libvpx-1,smarter/aom,mwgoldsmith/libvpx,GrokImageCompression/aom,thdav/aom,luctrudeau/aom,ShiftMediaProject/libvpx,jmvalin/aom,shacklettbp/aom,shacklettbp/aom,ittiamvpx/libvpx-1,webmproject/libvpx,ShiftMediaProject/libvpx,mwgoldsmith/libvpx,mwgoldsmith/vpx,mwgoldsmith/vpx,thdav/aom,webmproject/libvpx,smarter/aom,mbebenita/aom,shacklettbp/aom,smarter/aom,ShiftMediaProject/libvpx,smarter/aom,mwgoldsmith/libvpx,GrokImageCompression/aom,smarter/aom,thdav/aom,GrokImageCompression/aom