multimedia/{mplayer|mencoder}: Fix WITH_DEBUG build

Newer clang versions are not happy when passing -mcpu=<something> on
i386 and amd64. This fixes the construction of CFLAGS during configure
in the WITH_DEBUG case.
Also, as it turns out, there was a const ptr issue in
libmpcodecs/ad_spdif.c whith also needed to be fixed.

PR: 280818
Reported by: cryptogranny@gmail.com