bsd-user: Implement set_mcontext and get_ucontext_sigreturn for RISCV

bsd-user: Implement set_mcontext and get_ucontext_sigreturn for RISCV

Added implementations for 'set_mcontext' and 'get_ucontext_sigreturn'
functions for RISC-V architecture,
Both functions ensure that the CPU state and user context are properly
managed.

Signed-off-by: Mark Corbin <mark@dibsco.co.uk>
Signed-off-by: Warner Losh <imp@bsdimp.com>
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: <20240916155119.14610-17-itachis@FreeBSD.org>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>