PTRACE_GET_THREAD_AREA is not handled by amd64 linux syswrap, which leads
to false positive errors in 64 bits program ptrace-ing 32 bits processes.
For example, the below error was wrongly reported on GDB:
==25377== Conditional jump or move depends on uninitialised value(s)
==25377== at 0x8A1D7EC: td_thr_get_info (td_thr_get_info.c:35)
==25377== by 0x526819: thread_from_lwp(thread_info*, ptid_t) (linux-thread-db.c:417)
==25377== by 0x5281D4: thread_db_notice_clone(ptid_t, ptid_t) (linux-thread-db.c:442)
==25377== by 0x51773B: linux_handle_extended_wait(lwp_info*, int) (linux-nat.c:2027)
....
==25377== Uninitialised value was created by a stack allocation
==25377== at 0x69A360: x86_linux_get_thread_area(int, void*, unsigned int*) (x86-linux-nat.c:278)
Fix this by implementing PTRACE_GET|SET_THREAD_AREA on amd64.
d7d8231750