[asan] A different way of detectinb stack overflow.

Instead of checking stack limits that are not well defined for the main thread,
we rely on siginfo::si_code and distance from SP.


git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@201673 91177308-0d34-0410-b5e6-96231b3b80d8