RISC-V: Adding XTheadSync ISA extension

RISC-V: Adding XTheadSync ISA extension

This patch adds support for the XTheadSync ISA extension.
The patch uses the T-Head specific decoder and translation.

The implementation introduces a helper to execute synchronization tasks:
helper_tlb_flush_all() performs a synchronized TLB flush on all CPUs.

Co-developed-by: LIU Zhiwei <zhiwei_liu@linux.alibaba.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Signed-off-by: Christoph Müllner <christoph.muellner@vrull.eu>
Message-Id: <20230131202013.2541053-3-christoph.muellner@vrull.eu>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>