* Replace OQS_PORTABLE_BUILD with OQS_DIST_BUILD
Also introduces OQS_OPT_TARGET and removes OQS_USE_CPU_EXTENSIONS
* Only compile sha3 avx2 code on Linux|Darwin
* Use new ARCH_ARM[X] flags in SIKE CMakeLists
* Update test_portability and rename to test_distbuild
* Update documentation for building Windows AMD64 from Ubuntu Bionic
* Update scripts/build-android.sh
* More specific CMAKE_SYSTEM_PROCESSOR for rasppi toolchain
* CI: Use OQS_DIST_BUILD in some jobs
* Replace OQS_get_available_CPU_extensions by OQS_CPU_has_extension
* ARM64v8/ARM32v7 runtime cpu feature detection
* Compile-time detection of some ARM features
* Toolchain files to cross compile for ARM32v7 and ARM64v8
* Remove unnecessary references to CMAKE_BUILD_TYPE=Release
* Use OQS_DIST_BUILD=ON on Windows
