[asan] introduce run-time flag uar_stack_size_log to control the size of FakeStack; don't crash when the fake stack is exhausted, move some code to .cc file

git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@191510 91177308-0d34-0410-b5e6-96231b3b80d8