Check CMAKE_ANDROID_ARCH_ABI of native Android support of CMake

Qt 5.14.0 uses ANDROID_ABI [1] in generated cmake files. This requires android toolchain file of NDK.
Also it should be possible to use builtin support in CMake [2] instead of NDK toolchain file. CMake uses CMAKE_ANDROID_ARCH_ABI for the same usage.

Our work around in our cmake script:

SET(ANDROID_ABI ${CMAKE_ANDROID_ARCH_ABI})

It would be nice if the create_make.prf could check for CMAKE_ANDROID_ARCH_ABI variable, too.

[1] https://code.qt.io/cgit/qt/qtbase.git/tree/mkspecs/features/create_cmake.prf?h=5.14#n46

[2] https://cmake.org/cmake/help/v3.7/variable/CMAKE_ANDROID_ARCH_ABI.html#variable:CMAKE_ANDROID_ARCH_ABI