Work around clang/llvm compilation problems.

These patches will allow us to compile the whole AOSP with clang/llvm
before all the following problems are fixed.
* Suppress warnings on unused variables.
  * Remove unused static variables.
* Use only gcc to compile code that requires gcc atomic functions.
  They should be converted to standard atomic functions later.
* Suppress warnings on redefined typedefs.

BUG: 20765701
Change-Id: I19bc0a256c19502fabcabd1ca158637a8b5741a2