proposal: x/sys/unix: PtraceGetRegs should allow getting non-default registers

[mitar]

Currently unix.PtraceGetRegs only gets default set of registers, NT_PRSTATUS. It would be great if it would be possible to also get other sets. Probably we need another function for this. Example is that on arm, one needs to use NT_ARM_SYSTEM_CALL to get the system call register.

Similarly for setting registers.

Because ptracePtr is private, one has to duplicate it if you want to create a custom ptrace call.

[ianlancetaylor]

Changing PtraceGetRegs would break too much existing code. In any case, it has the wrong type for receiving the register values. As far as I can see for each processor we have to define a struct type for each register set we want to support.