scons: Revert LTO and partial linking for gcc >=8.1

scons: Revert LTO and partial linking for gcc >=8.1

This reverts commit f41abbdb5cf5c67233f3d730885d43517969afda,
"scons: Enable LTO and partial linking with gcc >= 8.1."

LTO and partial linking does not work on GCC 9.3 on Ubuntu 20.04 when
compiling gem5.fast. This error was exposed via the following command:

```
docker run -u $UID:$GID --volume $(pwd):/gem5 -w /gem5 --rm \
gcr.io/gem5-test/ubuntu-20.04_all-dependencies:latest scons \
build/MIPS/gem5.fast
```

The following error was received:

```
usr/bin/ld: cannot find lib.fo.partial.lto.o: No such file or directory
/usr/bin/ld: error: could not unlink output file
collect2: error: ld returned 1 exit status
scons: *** [build/MIPS/mem/ruby/system/lib.fo.partial] Error 1
```

Issue-on: https://gem5.atlassian.net/browse/GEM5-555
          https://gem5.atlassian.net/browse/GEM5-557
Change-Id: Id9e7fc81aec9f94524acc92c05aabdf96bd284cd
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/29272
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>