RISC-V: Add T-Head Fmv vendor extension

This patch adds the XTheadFmv extension, which allows to access the
upper 32 bits of a double-precision floating-point register in RV32.

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>