bsd-user: Add RISC-V thread setup and initialization support

bsd-user: Add RISC-V thread setup and initialization support

Implemented functions for setting up and initializing threads in the
RISC-V architecture.
The 'target_thread_set_upcall' function sets up the stack pointer,
program counter, and function argument for new threads.
The 'target_thread_init' function initializes thread registers based on
the provided image information.

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: <20240916155119.14610-10-itachis@FreeBSD.org>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>