[asan] simplify the code that compute the shadow offset; get rid of two internal flags that allowed to override it. The tests pass, but still this change might break asan on some platform not covered by tests. If you see this, please submit a fix with a test.

git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@202033 91177308-0d34-0410-b5e6-96231b3b80d8