Add PULP extensions and LLVM tablegen generation

[colluca]

• Mirror the ISA extension organization for CSRs (Mirror ISA extension organization for CSRs riscv/riscv-opcodes#422, initial review received but not merged upstream). Enables generating LLVM tablegen descriptions for the custom CSRs.

• Add --warn-overlap flag to CI tests.

• Clean up PULP extensions

  • Replace Xpulppostmod with Xcvmem
  • Remove all FPalt format instructions, which have the exact same encoding as non-alternate-format instructions
  • Split SmallFloat ISA extension into per-precision subextensions
  • Remove draft bit-manipulation instructions from rv32_xb that were since ratified in rv_zbb, rv64_zbb, rv_zba, rv_zbc and rv_zbkb
  • Remove draft half-precision SmallFloat instructions from rv_xsmallfloat_h that were since ratified in rv_zfhmin, rv_zfh, rv64_zfh and rv_d_zfhmin

• Add scripts to generate Tablegen inputs for custom LLVM 18 toolchains

• Update scripts to generate Tablegen inputs for custom LLVM 22 toolchains (on top of the prior so LLVM 18 can be still supported by forking off the prior commit)