bsd-user: Define RISC-V system call structures and constants

bsd-user: Define RISC-V system call structures and constants

Introduced definitions for the RISC-V system call interface, including
the 'target_pt_regs' structure that outlines the register storage
layout during a system call.
Added constants for hardware machine identifiers.

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>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-ID: <20240916155119.14610-12-itachis@FreeBSD.org>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>