scons: Track and reuse object nodes for a given source file.

scons: Track and reuse object nodes for a given source file.

scons gets upset if two different environments are used to set up a
particular object file. This change adds two dicts to the SourceFile
class, one for static and one for shared object files, which are keyed
off of the appropriate suffix. If a suffix hasn't been set up yet,
a new node of the appropriate type is set up and stored in the cache,
and then whatever is in the cache (new or old) is returned.

Change-Id: Ice4b4fc728b438a4d3316c3ff6667c0480d2a6d7
Reviewed-on: https://gem5-review.googlesource.com/6224
Reviewed-by: Nikos Nikoleris <nikos.nikoleris@arm.com>
Maintainer: Gabe Black <gabeblack@google.com>