Android: fix Android O version check for LLVM

Android: fix Android O version check for LLVM

With the release of O, the MESA_ANDROID_MAJOR_VERSION has changed to 8.
Change the LLVM check to match. There's no point to continue to support 'O'
as no one is going to use an old AOSP master.

Presumably, we'll be back here again to fix things again for P (or 9).

Reviewed-by: Chih-Wei Huang <cwhuang@linux.org.tw>
Signed-off-by: Rob Herring <robh@kernel.org>