RISC-V: Add generic support for vendor extensions

This patch introduces changes that allow the integration of vendor ISA
extensions:
* Define a list of vendor extensions (riscv_supported_vendor_x_ext)
  where vendor extensions can be added
* Introduce a section with a table in the documentation where vendor
  extensions can be added

To add a vendor extension that consists of instructions only,
the following things need to be done:
* Add the extension to the riscv_supported_vendor_x_ext list
* Add lookup entry in riscv_multi_subset_supports
* Documenting the extension in c-riscv.texti
* Add test cases for all instructions
* Add MATCH*/MASK* constants and DECLARE_INSN() for all instructions
* Add new instruction class to enum riscv_insn_class
* Define the instructions in riscv_opcodes
* Additional changes if necessary (depending on the instructions)

Co-developed-by: Lifang Xia <lifang_xia@linux.alibaba.com>
Signed-off-by: Christoph Müllner <christoph.muellner@vrull.eu>

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