target/riscv: Fix medeleg[11] read-only zero bit for M-mode ECALL

RISC-V Privileged Specification 3.1.8 (Machine Trap Delegation Registers (medeleg and mideleg)) mentions: "For exceptions that cannot occur in less privileged modes, the corresponding medeleg bits should be read-only zero. In particular, medeleg[11] is read-only zero." QEMU incorrectly included RISCV_EXCP_M_ECALL in DELEGABLE_EXCPS. It allowed the 11th bit to be written and read as set. Fixed by removing it from the DELEGABLE_EXCPS mask, adhering to the specification. Resolves: https://gitlab.com/qemu-project/qemu/-/work_items/3438 Signed-off-by: Abhigyan Kumar <314abh@gmail.com> Reviewed-by: Alistair Francis <alistair.francis@wdc.com> Message-ID: <20260427060849.749179-2-314abh@gmail.com> [ Changes by AF: - Remove comment ] Signed-off-by: Alistair Francis <alistair.francis@wdc.com>