Subzero: Build both Debug and Release version of llvm2ice.

Subzero: Build both Debug and Release version of llvm2ice.

Separate objects are built with -O0 and -O2.

Separate executables are built:
  build/Release/llvm2ice - Release build
  build/Debug/llvm2ice - Debug build

The executable built depends on whether the DEBUG make variable is set:
  make -f Makefile.standalone
  make -f Makefile.standalone DEBUG=1

The llvm2ice file in the top-level directory is always removed and symlinked to the appropriate build.

BUG= none
R=jvoung@chromium.org

Review URL: https://codereview.chromium.org/605093002