util: Split out channel-printing Python code

util: Split out channel-printing Python code

Rather than iterate over format.channels and format.swizzles directly,
use Python subfunctions that take the channel and swizzle lists as
arguments.  This allow the channel and swizzle lists to depend on
endianness.

There is no change to the generated u_format_table.c.

Signed-off-by: Richard Sandiford <rsandifo@linux.vnet.ibm.com>
Signed-off-by: José Fonseca <jfonseca@vmware.com>