Merge android-port & kernel-cache feature

Android port for merge

Major changes are in enabling kernel-cache
pocl_util.c : 
pocl_choose_dir() - chooses root folder for caching
pocl_create_source_dirs() - searches for matching program in sub-directories
other changes are either retaining files for caching or deleting unwanted files after cmpilation
use /sys/devices/system/cpu/possible file to find num-cores. Some android kernel shows no processor info on /proc/cpuinfo
plus pocl-android-sample in examples/
OK, I see. The lack of hashing requires one to create some kind of directory where to store the cached built kernels. Initially I thought this dir to be a hash of the preprocessed program source + build options etc., just like in ccache. Then it would not matter if the same kernel is accessed from multiple processes. You consider the hash function contributed by @clementleger too slow?