bsd-user: Implement 'get_mcontext' for RISC-V

bsd-user: Implement 'get_mcontext' for RISC-V

Added the 'get_mcontext' function to extract and populate
the RISC-V machine context from the CPU state.
This function is used to gather the current state of the
general-purpose registers and store it in a 'target_mcontext_'
structure.

Signed-off-by: Mark Corbin <mark@dibsco.co.uk>
Signed-off-by: Ajeet Singh <itachis@FreeBSD.org>
Signed-off-by: Warner Losh <imp@bsdimp.com>
Co-authored-by: Warner Losh <imp@bsdimp.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-ID: <20240916155119.14610-16-itachis@FreeBSD.org>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>