scons: Don't use isdir in AddLocalRPATH.

scons: Don't use isdir in AddLocalRPATH.

isdir isn't a nice way to check if an FS.Base is a File or a Dir as was
initially assumed, it literally checks if a path can be stat-ed and is
reported as a directory by stat. This means that if a directory is
going to be created as part of the build, the result of that test will
change depending on whether that part of the build has happened
successfully before.

A better check which behaves as originally intended is to check whether
the Node is an instance of the SCons.Node.FS.Dir class.

Change-Id: Id041917d50b768a8205769c0a05320f92b09993c
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/17128
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Maintainer: Gabe Black <gabeblack@google.com>