ff_aac_coder_init_mips() modifies a static const structure of
function pointers. This will crash if the binary uses relro
and is a data race in any case.

Furthermore it points to a maintainability issue: The
AACCoefficientsEncoder structures have been constified
in commit fd9212f2,
a Libav commit merged in 318778de.
Libav did not have the MIPS-specific AAC code and so this was
fine for them; yet FFmpeg had them, but this was not recognized.

Commit 75a099fc points to another
maintainability issue: Contrary to ordinary DSP code, this code
here is way more complex and needs to be constantly kept in sync
with the ordinary code which it mimicks and replaces. Said commit
is the only commit actually changing aaccoder.c in the last few
years and the same change has not been performed for the MIPS
clone; before that, it even happened several times that the mips
code was broken due to changes of the generic code (see commits
97437bd1 and
de262d01 or
860dbe02 or
933309a6 or
b65ffa31). This might even lead
to scenarios where someone changing non-dsp aacenc code would
have to modify mips inline asm in order to keep them in sync.
This is obviously a significant burden (if the AAC encoder were
actively developed).

Finally, the code does not even compile here due to errors like
"Error: float register should be even, was 1".
Reviewed-by: Lynne <dev@lynne.ee>
Reviewed-by: Jean-Baptiste Kempf <jb@videolan.org>
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>