configure: do not disable av_always_inline with --enable-small

Currently, --enable-small turns av_always_inline into plain inline,
which is more or less ignored by the compiler.  While the intent of
this is probably to reduce code size by avoiding some inlining, it
has more far-reaching effects.

We use av_always_inline in two situations:

1. The body of a function is smaller than the call overhead.
   Instances of these are abundant in libavutil, the bswap.h
   functions being good examples.

2. The function is a template relying on constant propagation
   through inlined calls for sane code generation.  These are
   often found in motion compensation code.

Both of these types of functions should be inlined even if targeting
small code size.

Although GCC has heuristics for detecting the first of these types,
it is not always reliable, especially when the function uses inline
assembler, which is often the reason for having those functions in
the first place, so making it explicit is generally a good idea.

The size increase from inlining template-type functions is usually
much smaller than it seems due to different branches being mutually
exclusive between the different invocations.  The dead branches can,
however, only be removed after inlining and constant propagation have
been performed, which means the initial cost estimate for inlining
these is much higher than is actually the case, resulting in GCC
often making bad choices if left to its own devices.

Furthermore, the GCC inliner limits how much it allows a function to
grow due to automatic inlining of calls, and this appears to not take
call overhead into account.  When nested inlining is used, the limit
may be hit before the innermost level is reached.  In some cases, this
has prevented inlining of type 1 functions as defined above, resulting
in significant performance loss.

Signed-off-by: Mans Rullgard <mans@mansr.com>
