Build for x86_64-linux-android with generated/unix/ code

[rib]

This enables building for x86_64-linux-android targets, which is particularly useful for running under an Android emulator.

For reference here I first explicitly ran structgen (built for x86_64-linux-android to with the latest r25 Android NDK toolchain) on an Android emulator and compared the output to the existing generated/unix/ code, which was identical.

This was how I double checked the output of structgen cross-compiled for x86_64-android-linux on an Android emulator:

export ANDROID_API_LEVEL=31
export ANDROID_ARCH=x86_64 # | i686
export ANDROID_ABI=x86_64 # | x86
export HOST_OS=linux # | windows
export EXT="" # | .bat

"$ANDROID_NDK_ROOT/toolchains/llvm/prebuilt/$HOST_OS-x86_64/bin/clang++" \
    -DANDROID \
    --target=$ANDROID_ARCH-linux-android$ANDROID_API_LEVEL \
    -std=c++14 \
    -DNDEBUG \
    -DPX_PHYSX_STATIC_LIB \
    -static \
    "-I" "physx/physx/include" \
    "-I" "physx/pxshared/include" \
    "-I" "physx/physx/source/foundation/include" \
    "-o" "./structgen" "src/structgen/structgen.cpp"

# Install emulator via Android SDK
$ANDROID_HOME/cmdline-tools/latest/bin/sdkmanager$EXT --update
$ANDROID_HOME/cmdline-tools/latest/bin/sdkmanager$EXT "system-images;android-$ANDROID_API_LEVEL;default;$ANDROID_ABI"
$ANDROID_HOME/cmdline-tools/latest/bin/avdmanager$EXT create avd -n pixel_4_$ANDROID_ABI -k "system-images;android-$ANDROID_API_LEVEL;default;$ANDROID_ABI" -d pixel_4

$ANDROID_HOME/emulator/emulator -avd pixel_4_$ANDROID_ABI -no-window

adb push ./structgen /data/local/tmp/structgen
adb shell "cd /data/local/tmp && ./structgen"
adb pull /data/local/tmp/{structgen_out.hpp,structgen_out.rs} .

[tgolsson]

Changelogs please, then I think we're G2G!