Snitch isa extensions

[igor-sachok]

Adds a parameterized XPULPv2 extension to Snitch. Each XPULPv2 sub-extension can now be enabled or disabled individually through the configuration file. An example configuration demonstrating these options is provided under xpulpv2.
Also introduces a private multiplier flag for the CC.
To use the corresponding LLVM model when building software, set SN_MCPU=snitch-mempool in the make sw command.
Tests for Snitch with XPULPv2 have been added to the sw/tests directory and verified through CI.

TODO

• Merge and bump riscv-opcodes (Update parse_opcodes for allow-overlap branch riscv-opcodes#16)
• Merge and bump llvm-project (snitch-mempool proc added llvm-project#32)
• Change parameterization of Mempool IPU vs. shared MULDIV unit

[colluca]

Thanks for the contribution!

I think it mostly looks good, but there are two major things to rework IMO:

• the shared vs. private muldiv configuration, and new vs. old IPU
• the organization of the SW tests

See detailed comments below.