nptl: Add tst-minstack-cancel, tst-minstack-exit [BZ #22636]

I verified that without the guard accounting change in commit
630f4cc3 (Fix stack guard size
accounting) and RTLD_NOW for libgcc_s introduced by commit
f993b875 (nptl: Open libgcc.so with
RTLD_NOW during pthread_cancel), the tst-minstack-cancel test fails on
an AVX-512F machine.  tst-minstack-exit still passes, and either of
the mentioned commit by itself frees sufficient stack space to make
tst-minstack-cancel pass, too.

Reviewed-by: Carlos O'Donell <carlos@redhat.com>
(cherry picked from commit d8b77890)