target/riscv: Fix write_misa vs aligned next_pc

target/riscv: Fix write_misa vs aligned next_pc

Do not examine a random host return address, but
properly compute the next pc for the guest cpu.

Fixes: f18637cd611 ("RISC-V: Add misa runtime write support")
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Daniel Henrique Barboza <dbarboza@ventanamicro.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Message-ID: <20250425152311.804338-8-richard.henderson@linaro.org>
[ Changes by AF:
 - Change `& ~3` to `& 3`
]
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>