libtheora: disable all 32-bit arm assembler

libtheora: disable all 32-bit arm assembler

The 32-bit Arm assembler is mostly broken. In 1.1.0 it was never used,
and 1.2.0 tries to enable it and there are a number of different ways it
can fail (some gcc/architecture combinations, and all clang builds).

Until this is fixed upstream, simply disable assembler entirely.

Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>