New crossfeed algorithm for Rockbox: "Meier" crossfeed

Emulates the basic "Meier" crossfeed (2 capacitors, 3 resistors)
as discussed in
http://www.meier-audio.homepage.t-online.de/passivefilter.htm

This crossfeed blends a bit of low-pass filtered L signal into
the R signal (and vice versa) while adding about 300 us delay
to the crossfed-signal. A difference with the crossfeed already
present in rockbox, is that this algorithm keeps the total
spectrum flat (the one currently in rockbox accentuates
low-frequency signals, making it sound a bit muffled).

This implementation is quite lightweight, just 3 multiplies per
left-right pair of samples. Has a default C implementation and
optimized assembly versions for ARM and Coldfire.

The crossfeed effect is quite subtle and is noticeable mostly
one albums that have very strong left-right separation (e.g.
one instrument only on the left, another only on the right).

In the user interface, the new crossfeed option appears as
"Meier" and is not configureable. The existing crossfeed is
renamed to "Custom" as it allows itself to be customised.

There is no entry for the user manual yet.

Change-Id: Iaa100616fe0fcd7e16f08cdb9a7f41501973eee1

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