opus: allow to select fixed-point implementation

The default of choosing the fixed-point implementation if
the target hardware does not have floating-point unit is not
always the best choice. Modern ARM hardware has floating
point support but most likely the fixed-point implementation
in Opus will still perform better. In particular if your CPU
supports the NEON extension.

This commit allows to select the fixed-point implementation
manually and sets the default based on the availability of
a floating-point unit.

[Peter: force option for sw float, slightly reword]
Signed-off-by: Sven Neumann <neumann@teufel.de>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>