config: Add --mem-size option in fs_bigLITTLE.py
config: Add --mem-size option in fs_bigLITTLE.py

Allow users to specify system memory size.

Change-Id: I1e0c099dc08d04f71b406d8cc86850f68b6048cb
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/20899
Tested-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
Reviewed-by: Giacomo Travaglini <8eef13a4f970e2e9372c8d26ad0737b760f99c02@arm.com>
Maintainer: Giacomo Travaglini <8eef13a4f970e2e9372c8d26ad0737b760f99c02@arm.com>

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