Fix nios2-linux-user syscalls. Fix nios2-linux-user sigreturn. Enable tests for nios2-linux-user. Remove special handling of SIGSEGV. Check supervisor for eret, bret. Split special registers out of env->regs[]. Clean up interrupt processing. Raise unaligned data and destination exceptions. Set TLBMISC fields correctly on exceptions. Prevent writes to read-only or reserved control fields. Use tcg_constant_tl(). Implement shadow register sets. Implement external interrupt controller interface. Implement vectored interrupt controller. Enable semihosting tests for nios2-softmmu.