Uses of a bit vector prevents generation of negative values.

[MikeOpenHWGroup]

In core-v-verif pull-request #2357 we discussed the need for negative values of a generated gpr value. The purpose of this issue is to ensure we don't forget about that discussion.

[XavierAubert]

As discussed with @pascalgouedo this morning, if we get that correctly, in this specific case, this only limits to locations set higher than 32'h8000_0000, instead of around 32'h8000_0000 as you ask, and thus, in the end, it doesn't quite impact the required behaviour, which is preventing stores to corrupt program data.

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