RISC-V: Add T-Head MemIdx vendor extension

T-Head has a range of vendor-specific instructions.
Therefore it makes sense to group them into smaller chunks
in form of vendor extensions.

This patch adds the XTheadMemIdx extension, a collection of T-Head specific
GPR memory access instructions.
The 'th' prefix and the "XTheadMemIdx" extension are documented in a PR
for the RISC-V toolchain conventions ([1]).

In total XTheadCmo introduces the following 44 instructions
(BU,HU,WU only for loads (zero-extend instead of sign-extend)):

* {L,S}{D,W,WU,H,HU,B,BU}{IA,IB} rd, rs1, imm5, imm2
* {L,S}R{D,W,WU,H,HU,B,BU} rd, rs1, rs2, imm2
* {L,S}UR{D,W,WU,H,HU,B,BU} rd, rs1, rs2, imm2

[1] https://github.com/riscv-non-isa/riscv-toolchain-conventions/pull/19



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