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
