Fix pthread_attr_getstack__main_thread.

There were two problems here:

* This would fail when run with unlimited stack, because it didn't know
  that bionic reports unlimited stacks as 8MiB.

* This would leave RLIMIT_STACK small, causing failures to exec (so the
  popen and system tests would fail).

Change-Id: I5b92dc64ca089400223b2d9a3743e9b9d57c1bc2