bsd-user: Define RISC-V VM parameters and helper functions

bsd-user: Define RISC-V VM parameters and helper functions

Added definitions for RISC-V VM parameters, including maximum and
default sizes for text, data, and stack, as well as address space
limits.
Implemented helper functions for retrieving and setting specific
values in the CPU state, such as stack pointer and return values.

Signed-off-by: Mark Corbin <mark@dibsco.co.uk>
Signed-off-by: Ajeet Singh <itachis@FreeBSD.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-ID: <20240916155119.14610-11-itachis@FreeBSD.org>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>