scons: Monkey patch a fix from 3.0.2 into 3.0.0 and 3.0.1.
scons: Monkey patch a fix from 3.0.2 into 3.0.0 and 3.0.1.

Ensure that SCons.Subst.Literal can be hashed.

https: //pairlist2.pair.net/pipermail/scons-dev/2018-October/004766.html
Change-Id: I5221b63a5fd63189eb690c77e780805dc6d68125
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/58633
Maintainer: Gabe Black <d3e8afea784ab4a50a2dbf9149d2eaa8478e1cbc@gmail.com>
Reviewed-by: Bobby Bruce <b9dba908ffb6c6d8ffe449cc88b3709c13c485c2@ucdavis.edu>
Maintainer: Bobby Bruce <b9dba908ffb6c6d8ffe449cc88b3709c13c485c2@ucdavis.edu>
Tested-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>

gem5/gem5,gem5/gem5,gem5/gem5,gem5/gem5,gem5/gem5,gem5/gem5,gem5/gem5