bsd-user: Define RISC-V register structures and register copying

bsd-user: Define RISC-V register structures and register copying

Added definitions for RISC-V register structures, including
general-purpose registers and floating-point registers, in
'target_arch_reg.h'. Implemented the 'target_copy_regs' function to
copy register values from the CPU state to the target register
structure, ensuring proper endianness handling using 'tswapreg'.

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: <20240916155119.14610-7-itachis@FreeBSD.org>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>