pixman-arma64: Adjustments to build with llvm
These syntactic changes enable building the arm64 neon assembly files with clang's integrated assembler. This seems to be the only way to get these optimizations onto Apple's arm64 devices like iPhones and Mac M1 at the time. See #46
Tests seem to run fine on Android arm64 devices - though I just ran some of them manually due to a lack of a proper build-environment there to just run the whole suite. Most notably the scaling-test which should utilize the bilinear-scaling codepath, runs without problems.
I cross-compiled the lib for arm64 iPhones but did not test the results yet as I do not have a native build-environment for Apple arm64 at hand and the test-suite needs libpng. I'll have some colleagues who use M1 macs run this in short future.
All in all I have to say the syntactic changes are quite ugly, but this way the code compiles with clang as well as with GAS.