update installation instructions for recent Android NDKs and support ARM64
Works for Android NDKs with unified headers (see https://android.googlesource.com/platform/ndk/+/ndk-release-r16/docs/UnifiedHeaders.md). Separate step of configuring a standalone toolchain is recommended by the Android docs. Explicitely setting CC to clang is useful, since gcc in Android NDK is deprecated and outright broken in NDK version 18. Using the bfd linker is needed for bdwgc to work correctly (see also https://github.com/ivmai/bdwgc/issues/259).
Showing with 88 additions and 18 deletions