[hwasan] Fix vfork handling with large stack limit.

Remove the maximum stack cleanup size check. With ulimit -s unlimited
main thread stack can be very large, but we don't really have a choice
other than cleaning all of it. It should be reasonably fast - hwasan
cleans large shadow ranges with a single madvise call.

This change fixes check-hwasan after ulimit -s unlimited.

llvm-svn: 355137