Maintain seperate directories for heterogeneous cpus
in multi-node clusters

1. Create device cache dir based on long_name(complete cpu name in cpuinfo)
instead of short_name. Clusters with different architectures/names cpus
will have seperate device directories

2. Rename temp_dir to cache_dir in program structure. Now that kernel cache is used
Capture build timestamp in cmake
This is a parameter for kernel cache
Rename function to something meaningful
Merge branch 'master' of github.com:krrishnarraj/pocl