target/riscv: Remove unconditional MENVCFG_CDE from mask

Currently, the MENVCFG_CDE (Counter Delegation Enable) bit is unconditionally included in the base write mask for CSR_MENVCFG. This make the subsequent conditional check `(cfg->ext_smcdeleg ? MENVCFG_CDE : 0)` completely ineffective, as a bitwise OR cannot clear a bit that is already set. Fix this by removing MENVCFG_CDE from the initial base mask. The bit will now only be writable when explicitly granted by the `ext_smcdeleg` configuration. This issue was discovered and reported by SpecHunter, an AI-driven architecture specification analysis tool. Link: https://github.com/yizishun/rv-isa-sec/blob/master/output/riscv-isa-manual/pr-2601/qemu.txt Signed-off-by: Zishun Yi <vulab@iscas.ac.cn> Reviewed-by: Daniel Henrique Barboza <daniel.barboza@oss.qualcomm.com> Message-ID: <20260511040534.2862443-1-vulab@iscas.ac.cn> Signed-off-by: Alistair Francis <alistair.francis@wdc.com>