avcenc: Clarify the ifdefs surrounding inline assembly

Previously, the code was structured like this:

#if defined(__GNUC__) && defined(__arm__)
    /* Generic C implementation */
#elif defined(__CC_ARM)
    /* RVCT version */
#elif defined(__GNUC__) && defined(__arm__)
    /* GCC arm assembly version */
#endif

This had two implications - the gcc arm version never was used
in practice, and the code required -D__arm__ to build at all for
other architectures.

The inline assembly constraints are buggy (requires changes so as
to not crash), and if fixed, the optimizations still aren't faster
than the generic C code on modern gcc versions. Therefore, just
ignore them and use the generic C code.

Due to the peculiar code block ordering and preprocessor conditions,
the generic C code is what has been used before anyway, but it
did require -D__arm__ to build, which can cause problems if building
for other architectures, if the system headers have ifdefs for this
define. This change makes it build without that define.

Change-Id: Ib3fcf5651ee720310b7dff10fd04279b5f90759c