bsd-user: Implement RISC-V CPU initialization and main loop

bsd-user: Implement RISC-V CPU initialization and main loop

Added the initial implementation for RISC-V CPU initialization and main
loop. This includes setting up the general-purpose registers and
program counter based on the provided target architecture definitions.

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: <20240916155119.14610-2-itachis@FreeBSD.org>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>