libgo, syscall: fix ptrace implementation on MIPS

libgo, syscall: fix ptrace implementation on MIPS

    On MIPS, the correct structure for PtraceRegs is 'struct pt_regs' which
    is declared in linux/ptrace.h. Previously no PtraceRegs structure was
    created on MIPS because 'struct user_regs_struct' doesn't exist there.

    Fallback to using pt_regs when the PtraceRegs structure is generated in
    mksysinfo.sh, then adjust syscall_linux_mipsx.go to read the program
    counter from the correct field.

    In addition, implement PtraceGetRegs and PtraceSetRegs on all 3 ABI
    variants.

    syscall_linux_mips64x.go can now be removed since the ptrace code on
    all 3 ABIs is identical.

    Reviewed-on: https://go-review.googlesource.com/46150

From-SVN: r249473