add pushfd, popfd, bt, cpuid

[LinusU]

From Claude Fable 5.1 (high):

Four entries off the todo list in the misc codegen, one commit each.

pushfd/popfd round-trip the flags word, keeping bits we don't model such as ID so the usual toggle-ID probe for cpuid support works. bt handles the register and immediate forms; the memory-operand-with-register-index form can address bits outside the operand, so it stays a todo. cpuid describes a plain Pentium with an FPU and nothing else, so programs probing for TSC or MMX take the paths the runtime implements.

Together these are the CPU-detection sequence MSVC-era games run at startup.