linux-user: enable tb unlinking when compiled with NPTL

Fixes receiving signals when guest code is being executed in a tight
loop. For an example, try interrupting the following code with ctrl-c.

http://nchipin.kos.to/test-loop.c

The tight loop is ofcourse brainless, but it is also exactly how the waitpid* testcases
are implemented.

Signed-off-by: Riku Voipio <riku.voipio@nokia.com>
Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>
