`DEBUG` has been defined by some projects as a preprocessor macro.
Rename to fix builds.

Fixes: b/149238449
Change-Id: I9a77321504c7097a5c59dde8b918b52650a90c0f
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/41050Reviewed-by: Alexis Hétu <sugoi@google.com>
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>
Tested-by: Ben Clayton <bclayton@google.com>