Debug builds of x86 gcc/clang can run out of register.
Previously NDEBUG or _DEBUG was used to detect a debug build.
But those macros are not set by gentoo builds.
This CL switches to the compiler predefine __OPTIMIZE__ which is
built into clang and gcc.

BUG=libyuv:602
TEST=untested
R=wangcheng@google.com

Review URL: https://codereview.chromium.org/2451503002 .