Modifications required in generate_all.py script

[ghost]

The generate_all.py script is there to generate tests for all instructions. However the script is not thorough enough to create all tests. Two points needs to be addressed regarding this:

1. All the instructions should be covered along with all its legal combinations. Therefore, a regressive test generation script is desired.
2. All tests covering all instructions with all its variants and configuration should be placed in a directory which have a structure consistent with riscv-tests. This includes all the test placed in one folder (rv64uv in this case) with a makfrag listing all those tests that can be included in test environment (riscv-test-env) later. Header files (whether scalar or vector) should be included in a single directory (rv64uv/macros) and no changes should have to be made in the environment itself (or at the worst, least changes are desired) that uses those tests after compiling according to any target, spike or DUT etc.

I'll try to submit a PR very soon to address this problem. I'd prefer if this generation script doesn't alter the current generate_all.py script, and can be used as a standalone script to address just this problem. I'd appreciate your comments on it.

Regards,
Quswar Abid