Some functions run out of registers when compiled for debug,
fpic, with stack frames on 32 bit x86 with clang.
Previously they were enabled based on _DEBUG but that macro
is not set in some build systems.  This CL adds DEBUG macro as
well to cover those environments.

R=harryjin@google.com
BUG=libyuv:532

Review URL: https://codereview.chromium.org/1517693005 .