Revert "Disable __longjmp_chk protection"

This reverts commit b73a3693.

This version of the check doesn't work with generic-gnu, and figuring
out the correct symbol version at configure time is probably more work
than this is worth. May revisit in the future.

Change-Id: I6c75e88bd3bd82a4b21e09a25780fe53aacb7d70