Update the makefile generator to support .S arm assembly files

These are built if ASM_ARCH is set to arm.
Hook up building arm assembly in the generic linux and android makefiles
Enable the android runtime CPU feature detection

Build and include the android cpufeatures library in libcommon.
Don't set -mfpu=neon while building the normal C++ files

This will make the normal C++ code not run on non-NEON devices at
all, making the runtime CPU feature detection pointless.

Adding -mfpu=neon to CFLAGS is not necessary, it's enough to
add it while building those individual .S files (via ASMFLAGS).
Add the APPLE_IOS define when building for iOS with make

Previously the define was only used when building from within the
xcode project files.

This enables the arm neon assembly.
Exclude assembly files that are used as headers

This avoids some warnings about object files not containing any
symbols.
Merge pull request #408 from mstorsjo/exclude-asm-headers

Exclude assembly files that are used as headers
Merge pull request #401 from mstorsjo/android-arm-assembly

Enable the arm assembly in android builds