[sanitizer] More accurate scanf parsing without GNU extensions.

In __isoc99_*scanf we don't have to worry about GNUisms, and can parse
%a accurately.

Patch by Jakub Jelinek.


git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@174969 91177308-0d34-0410-b5e6-96231b3b80d8