Looking at allowed_flags I see some x86, sparc, mips but no ARM flags. Using both vo-foobarencode ebuilds as examples, replace-flags and filter-flags cause build failures in the ARMV7 asm source tree by stripping *all* flags from the build. The workaround for now is "don't use those functions". Other than adding some suitable ARM flags, I don't have the rest of the answer (since I stopped sleeping 6 months ago I should probably not attempt a fix).
Created attachment 405250 [details] failed build (using replace-flags)
Created attachment 405252 [details] good build (avoids replace-flags) Also threw in QA exec warning fix suggested by igli.
please list the exact flags you want white listed