ntdll: Integrate with GDB using r_debug.
Effectively implementing the debugger interface as described in https://sourceware.org/git/?p=glibc.git;a=blob;f=elf/rtld-debugger-interface.txt
This is an actual attempt at having this upstream, split from !1074 which people have been finding useful for debugging Wine directly in GDB. This supports everything you would expect, from automatic symbol loading as well as cross-syscall backtraces with the new Python unwinder.