Factored out Linux proc file reading into separate class.

Both NativeProcessLinux (in llgs branch) and Linux Host.cpp had similar code to handle /proc
file reading.  I factored that out into a new Linux-specific ProcFileReader class and added a method
that the llgs branch will use for line-by-line parsing.

This change also adds numerous Linux-specific files to Xcode that were missing from the Xcode
project files.

Related to https://github.com/tfiala/lldb/issues/27