swscale: ppc: Hide arch-specific initialization details

Also give consistent names to init functions.
swscale: consistent names for arch-specific acceleration functions
swscale: Add some missing av_cold to arch-specific init functions
swscale: cosmetics: Drop silly camelCase from swScale function pointer name
arm: dcadsp: Move synth filter initialization to dcadsp file
arm: float_dsp: Propagate cpu_flags to vfp initialization function
arm: fmtconvert: Split armv6 fmtconvert code off from vfp code
cosmetics: Place arch initialization calls in alphabetical order
ppc: cosmetics: Consistently format CPU flag detection invocations
x86: Add and use more convenience macros to check CPU extension availability
x86: avcodec: Use convenience macros to check for CPU flags
x86: avcodec: Consistently structure CPU extension initialization
ppc: don't return a value from a function declared void

Signed-off-by: Martin Storsjö <martin@martin.st>