clang: clang 3.5: ffast-math flag forces funsafe-math-optimizations, it ignores -fno-unsafe-math-optimizations and other fno based math optimizations.

I prefer thus revert O3 and add relevant flags.

About munaligned-access, the target device must support unaligned access.

Change-Id: Iaf990c31d4fe1a5eafbddb7ed8e5e5bdf74a196f