support: Fix support_set_small_thread_stack_size to build on Hurd

PTHREAD_STACK_MIN comes from <limits.h>, so include it explicitly.
However, it is not defined on Hurd, so compensate for that as well.

Built on x86_64-linux-gnu, i686-linux-gnu, i686-gnu.

Change-Id: Ifacc888ef86731c2639721b0932ae59583bd6b3e
Reviewed-by: Christian Brauner <christian.brauner@ubuntu.com>