SCons: add code to provide a libm5 shared library.

SCons: add code to provide a libm5 shared library.
Targets look like libm5_debug.so.  This target can be dynamically
linked into another C++ program and provide just about all of the M5
features.  Additionally, this library is a standalone module that can
be imported into python with an "import libm5_debug" type command
line.