bsd-user: Define RISC-V signal handling structures and constants

bsd-user: Define RISC-V signal handling structures and constants

Added definitions for RISC-V signal handling, including structures
and constants for managing signal frames and context

Signed-off-by: Mark Corbin <mark@dibsco.co.uk>
Signed-off-by: Ajeet Singh <itachis@FreeBSD.org>
Co-authored-by: Warner Losh <imp@bsdimp.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-ID: <20240916155119.14610-14-itachis@FreeBSD.org>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>