[glibc] nptl: Add missing placeholder abi symbol from nanosleep move

[Joseph Myers]

On Thu, 7 Nov 2019, Florian Weimer wrote:

> The strong_alias is needed because some binutils version/target
> combinations do not support multiple versions for one symbol.

Note that it's possible to generate such aliases automatically using 
__COUNTER__.  See how the totalorder symbol versioning handles that; it 
would be nice to convert that into something general used directly in the 
definitions of versioned_symbol / compat_symbol (after which it would be 
possible to incrementally eliminate existing definitions of aliases in 
lots of source files that are only for the purpose of applying symbol 
versions).

-- 
Joseph S. Myers
joseph@codesourcery.com