[RFC] killing remaining USE___THREAD use

[Mike Frysinger <vapier at gentoo dot org>]

there seems to be unspoken history here (useless commit messages strike 
again).  so i'll just dump what i see based on git history & mailing lists.

USE___THREAD used to exist before d063d164335938d557460bebaa7cfe388157b627.  
before that, there was a tls.h header that did:
	#if !defined NOT_IN_libc || defined IS_IN_libpthread
	# define USE___THREAD 1
	#else
	# define USE___THREAD 0
	#endif
which is to say, define USE___THREAD in libc & libpthread code.  that commit 
in Sep 2005 went ahead and deleted that define everywhere else and made it so 
that USE___THREAD was assumed everywhere.

the trouble is that elf/rtld.c was left with USE___THREAD checks.  the mailing 
list has this thread where Roland said "maybe some day it will be useful".
	https://sourceware.org/ml/libc-alpha/2005-09/msg00058.html

Andreas cleaned things up in Sep 2011 in 
de82006d43e198fd162807c9adc720c7ebd728a3.

but Ulrich reverted it a few hours later w/no message in 
037e9fe21c92216ef7032ea2796781ec27ca182a.

i guess the preference was to keep the code, so Andreas cleaned it up slightly 
the same day in 995a80dfbcb443ead5aa22682c884ec5c827a2ea.

fast forward to today and -Wundef is complaining about it.  if the code is 
dead and has yet to be utilized in 8 years, can't we kill it ?  or, if it does 
serve an actual purpose, can someone describe what that is so we can update 
the defines accordingly ?  if it's for debugging, then let's rename it 
DEBUG_RLTD_TLS or some junk.
-mike

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