Remove parent's DEJAGNU environment variable

[a4lg]

make report / make check on riscv-gnu-toolchain depends on the default installation of DejaGnu's riscv-sim.exp.

If an user overrides the DejaGnu environment with custom riscv-sim.exp, many tricks used in the testing framework in riscv-gnu-toolchain stops working.

I noticed this issue while testing riscv-gnu-toolchain with an environment which I use custom DejaGnu configuration to test GCC (alone) with QEMU + custom arguments (written in $DIR/boards/riscv-sim.exp where DEJAGNU=$DIR/site.exp).

So, this commit removes parent's DEJAGNU environment variable by unexporting it.

Note that unexport is a feature of GNU Make and (before approving) check whether using a "not in POSIX make" feature is appropriate for this project.

[cmuellner]

I wonder who is setting DEJAGNU so that we have to unexport it.

If this is only needed in a specific local environment, then a local wrapper script that calls make would be the best place to handle this. I don't think that non-default environments need to be supported if this can easily be done outside of this repo.

So I think it might be better to add a subsection in README.md to document the requirement and how to address potential issues.

[cmuellner]

After revisiting this, I still prefer not to merge this PR because it adds code that only prevents issues in certain environments. This PR would also prevent overriding DEJAGNU for cases where this is needed (for whatever reasons—see also https://www.gnu.org/software/dejagnu/manual/Customizing-DejaGnu.html).

In this particular case, it might be best to place the unexport in a local wrapper script for building toolchains. Or setting the DEJAGNU variable could be done in a wrapper script that is used for the other mentioned use-case ("test GCC (alone) with QEMU + custom arguments").