contrib: remove gas-preprocessor.pl usage on iOS armv7

It works without on iOS arm64, it should work the same on armv7.

Draft on top of !2638 (merged) and !2649 (merged). If it works we can skip !2649 (merged) directly.

According to #24678 (closed) it should work with Clang 5 and above. Not sure there are iOS/XCode toolchain still available that can target iOS 9 with an older Clang. If necessary we can get the CLang version in a similar way as we get the MINGW_W64_VERSION.