bsd-user: Implement RISC-V sysarch system call emulation

bsd-user: Implement RISC-V sysarch system call emulation

Added the 'do_freebsd_arch_sysarch' function to emulate the 'sysarch'
system call for the RISC-V architecture.
Currently, this function returns '-TARGET_EOPNOTSUPP' to indicate that
the operation is not supported.

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: <20240916155119.14610-9-itachis@FreeBSD.org>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>