Allow building plain armeabi binaries for android

Building with "make OS=android APP_ABI=armeabi" will
produce arm binaries that will conform to the armeabi
ABI - not using any features outside of armv5te by
default (but still optionally using the NEON functions at
runtime if detected - even though such devices should rather
use the default armeabi-v7a build).
Make the processing lib use mutexes from WelsThreadLib from the common library

This requires always building the WelsMutex* functions,
even if MT_ENABLED isn't set.
Merge pull request #410 from mstorsjo/android-armeabi

Allow building plain armeabi binaries for android
Merge pull request #456 from mstorsjo/use-common-threadlib

Make the processing lib use mutexes from WelsThreadLib from the common library
Merge remote-tracking branch 'upstream/master' into travis.osx