Update VexRiscv to b6118e6cc2a3, sbt to 1.9.7, SpinalHDL to 1.9.4; make PMP plugin selection configurable

[lschuermann]

This updates the pinned VexRiscv commit to b6118e6cc2a3, which (amongst other things) integrates the changes

• PmpPluginOld: fix NAPOT address calculation overflow issue SpinalHDL/VexRiscv#374
• Rename PmpPlugin -> PmpPluginNapot, PmpPluginOld -> PmpPlugin SpinalHDL/VexRiscv#378

These changes fix a bug in the "PmpPluginOld" (which supports all PMP addressing modes of NA4, NAPOT, and TOR) relating to address calculation for NAPOT regions. Futhermore, they "undeprecate" the PmpPluginOld by renaming it to PmpPlugin, indicating its full-featuredness compared to the now-called PmpPluginNapot.

Shipping both of these plugins, upstream VexRiscv allows this repository to select the appropriate one depending on the particular user preferences. To this end, I introduce a pmpAddressMatchingModes switch, where users can indicate which modes shall be supported in their CPU. For the VexRiscv_Secure target, it now defaults to the full-featured PMP plugin, effectively reverting to the state of #9. Changing this to the PmpPluginNapot as done in #12 unexpectedly broke downstream consumers such as TockOS, which thus needed to build and ship their own CPU variant (tock/tock#2860). Thus, this change should allow OSes such as Tock, which require a spec-compliant PMP implementation, to revert back to the pre-built VexRiscv_Secure CPU target.

I can add a VexRiscv_SecureNapot target, to also retain the more optimized, but feature-limited and non-spec compliant version, if desired.

Furthermore, this PR updates the sbt version to 1.9.7, to work around an issue with using it with recent JREs ([error] [launcher] error during sbt launcher: java.lang.UnsupportedOperationException: The Security Manager is deprecated and will be removed in a future release), and updates the SpinalHDL versions to match those of the upstream VexRiscv repo.

[Dolu1990]

Thanks :)