[sanitizer] Fix Android build.

Android actually has "__unused" macro defined in <cdefs.h>.



git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@190160 91177308-0d34-0410-b5e6-96231b3b80d8
[sanitizer] Avoid including any system headers in the system-header-free part of the runtime library.


git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@190161 91177308-0d34-0410-b5e6-96231b3b80d8