avserver: Remove unused loadable module support

There is no record of this ever being used at all, anywhere,
since the feature was added in 2effd274466.

This gets rid of extra linker tricks just to support a feature
that isn't used, simplifying portability to other platforms.

Signed-off-by: Martin Storsjö <martin@martin.st>
Employ consistent LIBAV_COMPAT_ multiple inclusion guards in compat/

Also fix a comment and an #endif comment.
compat: Add missing license boilerplates
arm: Add VFP-accelerated version of synth_filter_float

               Before           After
               Mean    StdDev   Mean    StdDev  Change
This function   9295.0 114.9     4853.2 83.5    +91.5%
Overall        23699.8 397.6    19285.5 292.0   +22.9%
arm: Add VFP-accelerated version of int32_to_float_fmul_scalar

               Before           After
               Mean    StdDev   Mean    StdDev  Change
This function   1175.0   4.4      366.2  18.3   +220.8%
Overall        19285.5 292.0    18420.5 489.1     +4.7%
fmtconvert: Add a new method, int32_to_float_fmul_array8

This is similar to int32_to_float_fmul_scalar, but
loads a new scalar multiplier every 8 input samples.
This enables the use of much larger input arrays, which
is important for pipelining on some CPUs (such as
ARMv6).
dcadec: Use int32_to_float_fmul_array8
arm: Add VFP-accelerated version of int32_to_float_fmul_array8

               Before           After
               Mean    StdDev   Mean    StdDev  Change
This function    366.2  18.3      277.8  13.7   +31.9%
Overall        18420.5 489.1    17049.5 408.2    +8.0%
arm: Add VFP-accelerated version of imdct_half

               Before           After
               Mean    StdDev   Mean    StdDev  Change
This function   2653.0  28.5     1108.8  51.4   +139.3%
Overall        17049.5 408.2    15973.0 223.2     +6.7%
arm: Add VFP-accelerated version of dca_lfe_fir

               Before           After
               Mean    StdDev   Mean    StdDev  Change
This function    868.2  33.5      436.0  27.0   +99.1%
Overall        15973.0 223.2    15577.5  83.2    +2.5%
arm: Add VFP-accelerated version of fft16

               Before           After
               Mean    StdDev   Mean    StdDev  Change
This function   1389.3  4.2       967.8  35.1   +43.6%
Overall        15577.5 83.2     15400.0 336.4    +1.2%
dcadsp: Add a new method, qmf_32_subbands

This does most of the work formerly carried out by
the static function qmf_32_subbands() in dcadec.c.
arm: Add VFP-accelerated version of qmf_32_subbands

               Before           After
               Mean    StdDev   Mean    StdDev  Change
This function   1323.0  98.0      746.2  60.6   +77.3%
Overall        15400.0 336.4    14147.5 288.4    +8.9%