Drop using _tlsbase and _tlstop in favor of access via NtCurrentTeb.

Drop using _tlsbase and _tlstop in favor of access via NtCurrentTeb.

        * cygtls.h (_tlsbase): Remove.  Replace throughout with
        NtCurrentTeb()->Tib.StackBase.
        (_tlstop): Remove. Replace throughout with
        NtCurrentTeb()->Tib.StackLimit.
        * dcrt0.cc (child_info_fork::alloc_stack): Move definition of local
        teb variable up to be used throughout.
        * include/cygwin/config.h (__getreent): Use inline function on both
        architectures.

Signed-off-by: Corinna Vinschen <corinna@vinschen.de>