Add initial support for ptrace ARCH_PRCTL request.

This is required to debug multi-threaded programs with versions of GDB
that are built against old kernel headers:

    PTRACE_ARCH_PRCTL is obsolete since [Linux] 2.6.25, where the
    fs_base and gs_base fields of user_regs_struct can be used
    directly.

    -- gdb/amd64-linux-nat.c

Note: GDB uses only GET_{F,G}S requests, that's why SET_{F,G}S
requests are not supported yet, I don't know how to test them anyway.