contrib: soxr: don't enforce CMAKE_SYSTEM_NAME on android

CMake will infer the CMAKE_SYSTEM_NAME variable from the generated toolchain on Android, and setting it prevents CMake from finding which system it really is and believe it's Windows. Because PKG_CONFIG_PATH entries are separated by a semi-colon on Windows, it prevents finding libraries like libavcodec.

This commit might disable correct SIMD detection if CMAKE_ANDROID_ARCH_ABI is not correctly set or if CMAKE_SYSTEM_PROCESSOR has an invalid value with regard to android autodetection scripts in CMake (Android-Determine.cmake).