RISC-V: Add T-Head Int vendor extension

This patch adds the XTheadInt extension, which provides interrupt
stack management instructions.

The XTheadFmv extension is documented in the RISC-V toolchain
contentions:
  https://github.com/riscv-non-isa/riscv-toolchain-conventions

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