bsd-user: Add RISC-V CPU execution loop and syscall handling

bsd-user: Add RISC-V CPU execution loop and syscall handling

Implemented the RISC-V CPU execution loop, including handling various
exceptions and system calls. The loop continuously executes CPU
instructions,processes exceptions, and handles system calls by invoking
FreeBSD syscall handlers.

Signed-off-by: Mark Corbin <mark@dibsco.co.uk>
Signed-off-by: Ajeet Singh <itachis@FreeBSD.org>
Co-authored-by: Jessica Clarke <jrtc27@jrtc27.com>
Co-authored-by: Kyle Evans <kevans@FreeBSD.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-ID: <20240916155119.14610-3-itachis@FreeBSD.org>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>