nptl: Open libgcc.so with RTLD_NOW during pthread_cancel [BZ #22636]

nptl: Open libgcc.so with RTLD_NOW during pthread_cancel [BZ #22636]

Disabling lazy binding reduces stack usage during unwinding.

Note that RTLD_NOW only makes a difference if libgcc.so has not
already been loaded, so this is only a partial fix.

Reviewed-by: Adhemerval Zanella <adhemerval.zanella@linaro.org>
(cherry picked from commit f993b8754080ac7572b692870e926d8b493db16c)