bsd-user: Add generic RISC-V64 target definitions

bsd-user: Add generic RISC-V64 target definitions

Added a generic definition for RISC-V64 target-specific details.
Implemented the 'regpairs_aligned' function,which returns 'false'
to indicate that register pairs are not aligned in the RISC-V64 ABI.

Signed-off-by: Warner Losh <imp@bsdimp.com>
Signed-off-by: Ajeet Singh <itachis@FreeBSD.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-ID: <20240916155119.14610-13-itachis@FreeBSD.org>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>