Enables building examples with Android NDK

Enables building examples with Android NDK

Soft enable runtime cpu detect for armv7-android target, so that it
can be disabled and remove dependency on 'cpufeatures' lib.
Change the arm_cpu_caps implementation selection such that 'no rtcd' takes
precedence over system type.

Switch to use -mtune instead of -mcpu. NDK was complaining about
-mcpu=cortex-a8 conflicting with -march=armv7-a, not sure why.

Add a linker flag to fix some cortex-a8 bug, as suggested by NDK Dev
Guide.

Examples:
Configure for armv7+neon:

./configure --target=armv7-android-gcc \
            --sdk-path=/path/to/android/ndk \
            --disable-runtime-cpu-detect \
            --enable-realtime-only \
            --disable-unit-tests

...armv7 w/o neon:

./configure --target=armv7-android-gcc \
            --sdk-path=/path/to/android/ndk \
            --disable-runtime-cpu-detect \
            --enable-realtime-only \
            --disable-neon \
            --cpu=cortex-a9 \
            --disable-unit-tests

Change-Id: I37e2c0592745208979deec38f7658378d4bd6cfa