Fix signed integer overflow in RV32M

[visitorckw]

The current implementation of the mul instruction does not guard against integer overflow, potentially leading to undefined behavior. Cast the operands to int64_t before performing the multiplication to ensure that the result can be accommodated without overflow. The lower 32 bits of the product are then extracted, preserving the correct uint32_t type.

[jserv]

Thank @visitorckw for contributing!